Physiology: The Science of the Body
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Title: Physiology: The Science of the Body
Author: Ernest G. Martin
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Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK PHYSIOLOGY: THE SCIENCE OF THE BODY ***
[Illustration: _Muscular Coordination_
_Weissmuller’s horizontal plunge into a swimming pool._]
POPULAR SCIENCE LIBRARY
EDITOR-IN-CHIEF
GARRETT P. SERVISS
AUTHORS
WILLIAM J. MILLER · HIPPOLYTE GRUENER · A. RUSSELL BOND
D. W. HERING LOOMIS HAVEMEYER · ERNEST G. MARTIN
ARTHUR SELWYN-BROWN · ROBERT CHENAULT GIVLER
ERNEST INGERSOLL · WILFRED MASON BARTON
WILLIAM B. SCOTT · ERNEST J. STREUBEL
NORMAN TAYLOR · DAVID TODD
CHARLES FITZHUGH TALMAN
ROBIN BEACH
ARRANGED IN SIXTEEN VOLUMES
WITH A HISTORY OF SCIENCE, GLOSSARIES
AND A GENERAL INDEX
_ILLUSTRATED_
[Illustration]
VOLUME NINE
P. F. COLLIER & SON COMPANY
NEW YORK
Copyright 1922
BY P. F. COLLIER & SON COMPANY
MANUFACTURED IN U. S. A.
PHYSIOLOGY
THE SCIENCE OF THE BODY
BY
ERNEST G. MARTIN
PROFESSOR OF PHYSIOLOGY, STANFORD UNIVERSITY
[Illustration]
P. F. COLLIER & SON COMPANY
NEW YORK
PREFACE
When Alexander Pope wrote “The proper study of mankind is man,” he was
thinking rather of man as a social being than as the possessor of an
amazingly complex and interesting body. It is nevertheless true that to
one who finds enjoyment in the study of intricate mechanisms or to one
for whom that amazing sequence of events which we call life has appeal
there is no more fascinating study than the study of the living body.
That part of the study of the body which concerns itself primarily with
activity and only secondarily with form and structure, makes up the
science of Physiology. The way the body works is the central theme.
The practical value of Physiology to the general reader lies in the fact
that it forms the basis of all sound rules of hygiene. Life is made up
of bodily activities which may be carried on correctly or incorrectly.
Carried on correctly they mean health, carried on incorrectly, unhealth.
The world is flooded with health-preserving or health-restoring systems,
urged upon the public, for the most part, by promoters in search of
gain. Such of these as have merit are based on definite physiological
principles, and anyone who has a common-sense working knowledge of his
own body can order his life in accordance with them, at little or no
expense. Moreover, a sound appreciation of Physiology drives home the
truth that when the body is really out of order its restoration can be
safely intrusted only to the properly trained physician: the man or
woman who through years of painstaking study has won insight into the
intricacies of the human mechanism and whose honest appreciation of the
difficulties of his profession, and courageous sincerity in grappling
with them, justify to the full the confidence in which he is held by his
community.
ERNEST G. MARTIN.
CONTENTS
CHAPTER PAGE
I. THE SIGNS OF LIFE 9
II. THE MAINTAINING OF LIFE 18
III. THE SOURCES OF FOOD 24
IV. THE USES OF FOOD 31
V. BODY CELLS 41
VI. THE SUPPORTING FRAMEWORK 53
VII. MOTION 73
VIII. SENSATION--INTERNAL AND CONTACT SENSES 86
IX. SENSATION--DISTANCE SENSES 96
X. THE NERVOUS SYSTEM AND SIMPLE NERVOUS
ACTIONS 122
XI. THE BRAIN AND COMPLEX NERVOUS ACTIONS 139
XII. SOME SPECIAL NERVOUS ACTIONS; SMOOTH
MUSCLE AND GLAND CONTROL 155
XIII. THE BODY FLUIDS 173
XIV. THE CONVEYER SYSTEM OF THE BODY 191
XV. THE SERVICE OF SUPPLY OF FOOD 226
XVI. THE SERVICE OF SUPPLY OF OXYGEN 253
XVII. THE SERVICE OF REMOVAL OF WASTE 271
XVIII. MORE ABOUT THE USE OF FOOD BY THE BODY 277
XIX. KEEPING WARM IN WINTER AND COOL IN SUMMER 305
XX. THE PERPETUATION OF THE RACE 324
XXI. CHILD AND MAN 345
LIST OF ILLUSTRATIONS
MUSCULAR COORDINATION--WEISSMULLER’S HORIZONTAL
PLUNGE INTO A SWIMMING POOL _Frontispiece_
FACING PAGE
FACTORY’S COOPERATION IN AIDING PHYSICAL FITNESS,
UNDERWEIGHT EMPLOYES ARE GIVEN A MILK LUNCH 32
WEIGHT TEST IN A CHILD’S PHYSICAL EXAMINATION TO
DETERMINE HOW NEARLY THE CHILD APPROACHES
CORRECT STANDARDS OF PHYSICAL DEVELOPMENT 33
FRENCH METHOD OF MEASURING VERTICAL CONFORMATION,
CONSIDERED VALUABLE IN ADVISING AND
TRAINING ATHLETES 64
CURVE OF THE SPINE MEASURED AND DRAWN FOR STUDY
OF AN ATHLETE’S POSSIBILITIES 65
MUSCULAR DEVELOPMENT OF AN ATHLETE--THE DISCUS
THROWER OF MYRON 80
A MODERN “VICTORY”--MISS SABIE AT PRACTICE 81
HOSPITAL DESIGNED TO SUPPLY THE MAXIMUM OF LIGHT
AND AIR--CHIEF REQUISITES FOR HEALTH--TO EACH
ROOM 96
ARCHITECT’S PLAN MADE WITH THE NEED FOR LIGHT
AND AIR IN MIND 97
SOFT, RESTFUL COLORS OF A HOSPITAL ROOM 112
MAGNET PUT TO THE SERVICE OF REMOVING IRON FILINGS
FROM THE EYE 113
TEST FOR BLOOD PRESSURE 192
INDICAN TEST IN URINALYSIS 193
MICROSCOPIC STUDY OF THE BLOOD 208
MICROPHOTOGRAPH OF BRAIN TISSUE 209
X-RAYING THE TEETH AND JAW--HEALTH OFTEN DEPENDS
ON THEIR GOOD CONDITION 224
THROAT EXAMINATION--IMPORTANT IN DETERMINING
PHYSICAL CONDITION 225
LUNCH HOUR IN FRENCH SCHOOL WHERE EXERCISE,
OUT-OF-DOOR LIFE, SUNSHINE AND FRESH AIR ARE
CONSIDERED ESSENTIALS 288
DIET KITCHEN ON A U. S. HOSPITAL SHIP 289
CARING FOR THE EARS--IMPORTANT FOR ADULTS AND
BABIES ALIKE 336
BABY’S FOOTPRINT--A MEANS OF POSITIVE IDENTIFICATION 336
BABIES, PHYSICALLY AND MENTALLY ACTIVE 337
CHAPTER I
THE SIGNS OF LIFE
Physiology is the study of living things, so the first thing to be asked
when we begin to think about physiology is how we are to know whether
anything is alive or not. It is usually pretty easy to tell whether a
dog or cat is alive or dead, although sometimes when a dog is stretched
out on the road we have to look closely to tell whether he has already
met his end or is merely courting it by sleeping in the public highway.
There are in the world hosts of animals with which we are not familiar,
and to tell whether these are alive or dead is often a puzzle. More than
one picnicker has been thoroughly surprised by seeing what looked like a
bit of dead twig begin to walk away, and recognized the walking stick.
On the whole we will agree that the sign of life which we find most
reliable is motion of some sort on the part of the living animal. If the
stretched-out dog makes breathing movements, we pronounce him alive; if
not, we decide that he is dead. It is because the walking stick moves
off when disturbed that we know it is not a twig. But while motion is
the thing we look for in living animals we know perfectly well that it
would be foolish to assert that anything that moves is alive. When the
wind is blowing the air may be full of dead leaves and butterflies, all
moving, but only part, the butterflies, alive. Unless the motion is
produced by the animal itself it is not good as a sign of life. So
widespread among animals is the making of movements, either on their own
account, or when disturbed, that we shall not often find ourselves
mistaken if we decide that an object which remains quiet indefinitely is
not a living animal. Now how about the other side of the question? Is
anything that moves on its own account or when disturbed to be judged
alive?
Suppose that the inhabitants of Mars have finally succeeded in
perfecting a flying boat that can be hermetically sealed and shot across
space from that planet to our earth. Suppose further, that the first
exploring party has set forth on a voyage of discovery, and has reached
a point high in our sky from which objects on the earth’s surface begin
to be distinguishable. Of course the huge landmarks, cities, lakes, and
rivers, have been in view for a long while, and now the explorers are on
the lookout for signs of living things. They are watching, just as we
would be, for moving objects. The first moving thing that they see will
probably be a train, and we can imagine their speculations as to whether
they are actually looking or not at an inhabitant of the earth. As their
craft sinks toward the surface the make-up of the train becomes
perceptible as also the fact that it runs on rails, showing that it is a
mechanical contrivance and not a living being. As smaller objects come
into view black, shiny specks are seen moving about. These show every
appearance of life; they start and stop; pass each other without
interference; in fact conduct themselves about as animals usually do. If
their apparent great power has the effect of discouraging the exploring
party, so that they give up further investigation and fly away to Mars,
the inhabitants of that planet will always suppose the earth to be
populated by automobiles. We know that automobiles are not alive, yet,
as this little allegory shows, they behave enough like living beings to
deceive distant observers. There must be some sign of life which will
apply to an animal and not to an automobile; what is it? Evidently what
the Martian explorers missed was the fact that the automobile does not
really start or stop itself, or guide itself past obstructions. If it
had been alive, it would have done these things of itself. It is not so
much the power of motion, then, that proves that the thing is alive as
the power of making motions that are under the control of the animal
itself.
The sight of an automobile which is not alive behaving as though it were
because it is under control of a driver who is alive may lead us to ask
whether the animal that we know to be living is actually alive in all
its parts, or is a dead mechanism of some sort which has somewhere
within it a living controller, corresponding to the living driver of the
car. The animals with which we are most familiar are ourselves; how is
it with our own bodies? Are they alive in all their parts, or is the
brain the only part of us which is living? When a patient goes under
ether on the operating table, or even when he is sound asleep, the signs
of life are not conspicuously present; the heart goes on beating, to be
sure, but so does the engine of an automobile go on running when the
driver is away, provided he has not shut it off. A favorite belief among
the Hindus is that when they go into a trance the body actually becomes
lifeless while the living spirit soars among the heights. How are we to
decide whether the Hindus are right or not? Evidently we shall have to
look deeper than we have thus far, and learn something of what is
actually going on in the different parts of our bodies when we are
asleep and when we are awake.
Nearly everyone learns in school the main facts about the construction
of the body; that there is a bony skeleton which supports the softer
parts; that motions are made by muscles; that sense organs inform us as
to what is going on in the world around; that the brain is the seat of
the mind; that heart, lungs, stomach, kidneys, and other organs
contribute in various ways to our well-being. Not so many go into detail
as to the make-up of these organs, or into the way in which they do
their work. This is not a simple matter, for several reasons. The first
is that the construction units are so tiny that they cannot be seen by
the unaided eye, but must be studied under the high magnification of a
first-class microscope. It is much harder to make out the manner of the
working of tiny pieces of machinery than of those that are of convenient
size. When the parts are as small as those that make up our bodies, the
task of finding out how they operate is so difficult that even now,
after years of study, there are many details about which we know very
little.
The construction units have been named cells. In some tiny animals the
whole body consists of but one cell; all higher animals, including
ourselves, have millions of cells making up the body. Undoubtedly some
cells are alive; our question is as to whether all of them are, or
whether there are some that are alive and some that are not. There are
parts of our bodies, and of the bodies of nearly all other kinds of
animals, as well, that are certainly not alive. Examples are the hair,
the nails, the enamel of the teeth, and the hard parts of the bones.
Actual living stuff is very soft and liquid. It is too fragile to hold
its own structure except in the very tiniest animals; those that are
larger need some additional supporting framework. In a body the size of
a man’s the supporting framework amounts to a very considerable
percentage of the entire weight (25 per cent). Not only is there the
large bony skeleton, but between and among the individual cells is a
framework made up of fine fibers and sheets which hold the cells in
place. This latter framework is called connective tissue; we run across
it in the gristly parts of meat. It makes up the stringy mass that
clings to the cutter of the meat grinder when beef is being ground for
Hamburg steak. We shall consider later how all this supporting material
is made and put in place. Just now we are interested in the cells, and
in determining whether all of them are alive or not.
There are many different kinds of cells in the body; some are muscle
cells, others nerve cells, still others gland cells, and so on. Careful
study shows, however, that at bottom all cells are alike. All are
composed of one kind of substance to which has been given the name of
protoplasm, meaning first or primary flesh. It is because some, at
least, of this protoplasm is alive that our bodies are alive, and our
physical life consists of nothing more than the combined life of all the
living protoplasm which our bodies contain. Is there any way by which to
tell whether any particular mass of protoplasm is alive or not? In other
words, what are the signs of life of protoplasm as contrasted with the
signs of life of whole animals?
We shall scarcely expect it to be as simple a matter to tell whether the
tiny mass of protoplasm that we call a cell is alive or not as to decide
whether a dog is dead or alive. For one thing, our most useful test of
life, namely motion, cannot always be applied to single cells. We have
in our bodies a great many cells, those in the brain, that we know are
alive if any part of us is, but aside from the exceedingly gradual
shifts in position that take place during growth the brain cells never
make any motions at all, so far as anyone has ever been able to find
out. Of course in the body of any ordinary animal most of the cells are
hidden from view beneath the skin, but there are enough small
transparent animals whose internal parts can be watched through the
microscope to let us say with certainty that some of the cells which we
know to be alive do not move. Tests of life that can be applied to all
kinds of cells will necessarily be difficult to use, and we shall have
to take the word of experts as to whether they have found particular
cells alive or not, but the principle on which the tests are based is
simple enough so that we can examine it. To do this, it will be
necessary to turn our attention for a little while to some of the very
tiniest of all living animals, those whose whole bodies consist of but
one cell.
When these tiny one-celled animals are watched through the microscope as
they swim about it can be seen that in one important feature they behave
just as we do ourselves; that is in their care not to neglect mealtime.
To be sure, mealtime comes for them whenever they happen to hit against
any tinier particle than themselves, which they can take in and digest.
But for them, as for us, the taking of food from time to time is a
necessity of life. Only a small part of the food thus taken in is added
permanently to the bulk of the animal. In other words, the growth does
not go on as fast as does the taking of food. Of course in ourselves,
after we have reached full size, there is little or no increase in
permanent bulk even though we do keep right on eating. Evidently in the
tiny one-celled animal, and in us as well, food is constantly being used
for something besides growth. It can be proved that this food is used
for precisely the same purpose that gasoline is used in the automobile,
namely to run the machine. In a very real sense every living thing is a
machine, and will no more run without a supply of power than will any
other machine. From the engineering standpoint animals can be classified
along with automobiles and locomotives as “prime movers,” namely, as
machines which develop their power within themselves. There are not many
kinds of power which can be developed by prime movers. By far the
commonest is that seen in locomotives and automobiles, namely the
burning of some kind of fuel. We have always known that the locomotive
operates by the burning of coal or oil in the fire box. A moment’s
thought will show us, if we have not realized it before, that the
explosion of the air-gas mixture in the automobile cylinders is also a
burning. Every steam-driven power plant depends on burning fuel for its
power. Evidently fuel materials contain abundant power, if it can be
extracted, and burning is a good method for doing the extracting. The
word “burning” is the common name for a chemical process known
technically as “oxidation,” meaning the union of oxygen with the fuel.
The air is one-fifth oxygen, so there is plenty available, and fuel will
usually oxidize readily after it is properly started.
Not only do animals correspond with other machines in using fuel as
their source of power; they correspond also in that the power is
extracted through the process of oxidation. To be sure, the oxidation in
animals is not accompanied by flame and smoke as it usually is in power
plants, nor do any parts of the animal get as hot as does the furnace
where fuel is ordinarily burned; but in spite of these differences the
fundamental fact is the same, namely that the extraction of power is by
means of oxidation. What this shows is that great heat, flame, and smoke
are not necessary in oxidation, but only in the kinds of oxidation with
which we are most familiar.
As soon as we have described one more feature of animal power
development, we shall be ready to apply what has been said to the topic
in hand, namely the signs of life in single cells. The point that
remains to be made is that in living cells power development has to go
on all the time whether the cell is active or not. This means that fuel
is constantly being burned, and oxygen is constantly being taken in to
do the burning. There has been, and still is, a great deal of debate as
to how much the oxidation can be reduced in living cells without
destroying life. It is evident that it can be cut down to a very low
level indeed, for seeds remain alive for years without using up, or even
noticeably depleting, the store of fuel material which they contain.
Most botanists of the present time doubt the truth of the tale that
grains of wheat have sprouted after being taken from the wrappings of
mummies, where they had lain for thousands of years. Careful efforts
have been made to preserve wheat under as favorable conditions as
existed in the mummy wrappings, but in every case the power to sprout
was lost within a comparatively few years. So far as experiment enables
us to judge, the complete cessation of power development in cells,
either of plants or animals, means their death.
Here we have our sign of life that is applicable to all kinds of cells
wherever they are located, whether making up the whole of a microscopic
animal or deeply imbedded in the body of a large animal which consists
of millions of them. If power development is going on, the cell is
alive; if no power is being developed, the cell is not alive. When this
test is applied it is found that all the protoplasmic cell masses which
are present in the body of a plant or animal are alive, and since such
masses are everywhere throughout the body, life is present in all parts
of it, and not confined to the brain or to any other single region. We
might admit that the Hindus are correct in assuming that the spirit can
sometimes soar away and leave the lifeless body behind, but we cannot
accept the possibility that it can return and establish life within it
again. When life is resumed after a trance, that fact is proof positive
that life continued throughout the trance itself.
CHAPTER II
THE MAINTAINING OF LIFE
Equal in importance to being alive is the power to go on living;
therefore, having described the signs of life, our next task is to
consider how that life is maintained. When the primary fact of life was
given as continuous power development, the foundation was laid for this
topic, for life cannot fail to go on if continuous power development is
maintained.
Power development in living animals as in locomotives depends on fuel
and oxygen; evidently continuous supplies of these must be provided if
life is to go on. The living animal differs from the locomotive in this:
that while some one attends to supplying the locomotive with fuel, most
living animals, except the very young, have to attend to providing
themselves. There are exceptions to this rule. The tapeworms that
inhabit the intestines of animals, and sometimes of men, live in a
stream of food; they are put to no trouble to obtain it. The same is
true of many kinds of parasites. Except for these, however, it holds
true that animals must attend to their own wants. We shall now begin to
see the utility of the most conspicuous sign of life spoken of in the
first chapter, namely, motion, for food must be gotten where it is; only
tapeworms and similar animals swim in it. All the rest, including
ourselves, must go to where the food is. Even animals like oysters,
that are anchored to the rocks, have to use motion in getting food. In
their case the motion consists in setting up a current in the sea water
into and through their bodies, from which current they sift out the tiny
food particles which abound in the ocean.
If an animal happens to live in the ocean, where every drop furnishes
its particle or two of foodstuff, and especially if the animal is small,
or sluggish, like the oyster, almost any kind of motion will serve to
bring the animal all the food it needs. The simplest of the one-celled
animals, that must be watched through the microscope to see how they
behave, blunder about aimlessly, and in the course of their blundering
bump up against food particles often enough to keep themselves fed. If
an animal happens to live where food is scarcer, or if it is big and
active, and so must have large quantities of food, aimless blundering
about will never get it enough to keep it alive. It must have some means
of finding out where the food is. Since we ourselves come under the head
of animals whose food needs are so large that we must locate food
supplies, and not depend on happening onto them, we can identify in
ourselves the means which are used for doing this. We all know that our
sense organs, the eyes, ears, nose, and finger tips are what we depend
on for telling us where food is to be found. The same is true of all
animals that are able to hunt for food; they have some sort of sense
organs to help in guiding them to where the food is.
The story of the machinery for finding food is not yet quite complete,
for the muscles which actually make the movements by which the animal
gets about are in one part of the body, while the sense organs which
are to furnish the information by which the movements are guided are in
quite a different part, and in animals as large as ourselves, some
distance away. From our eyes to our leg muscles is quite a space, and it
is evident that this space must be bridged somehow if our legs are to
move in obedience to information which our eyes bring in. In ourselves
and in almost all other animals this space is bridged by means of
special machinery for the purpose. We are familiar with it under the
name of the nervous system. We may not have been in the habit of
thinking of the nervous system in just this way, but at bottom this is
exactly what the nervous system does for us, namely, guides our muscles
according to the information brought in by our sense organs. There is
more to nervous activity than just this, but this is the starting point
and groundwork for all the rest, as we shall try to show presently.
Continuous food supplies are the main necessity for continued life, but
there are some other things that have to be looked out for in addition.
The favorite food for large numbers of animals, and, indeed, in many
cases the only food, consists of the bodies of other animals. All the
flesh-eating sorts prey on other animals for their food. This places the
other animals on the defensive, so that a large part of their activity
consists in escaping the attacks of the beasts that wish to eat them.
For most kinds of animals the greater part of their waking life is taken
up with movements which serve either to get them food or to prevent them
from becoming food for others. If we add to these the movements that are
necessary to preserve the animals against other kinds of danger than the
danger of being eaten, and those connected with the propagation and
care of the young, we shall have about covered the list of what we may
call the serious activities of animals, and of men as well. Many kinds
are active besides in play. This is particularly true of young animals,
although grown-ups, both among animals and men, find play both agreeable
and beneficial when not overindulged.
Protective motions need to be even more accurately made than those whose
purpose is the getting of food, for if the food is missed at one effort
another trial may be more successful, but if an attempt to escape fails
there will probably be no more chances to try. The sense organs and the
nervous system are just as deeply concerned, therefore, in avoiding harm
as in finding food materials, and it is as important for them to do
their work well in the one case as in the other. When we think of the
activities of animals, for whatever purpose they are carried on, we must
think of them as made up of the combined actions of the muscles, the
nerves, and the sense organs, and not of any of them working by
themselves.
These parts of us that are so closely concerned in the maintaining of
our life by getting us food and keeping us safe from harm make up, also,
the only parts of us which really share in what we may call conscious
living. When we come right down to it we could spare our other
organs--heart, lungs, stomach, and the rest--and never miss them so far
as adding anything to our happiness is concerned. In fact, the less
these organs intrude themselves into our attention the better off we
are; only when we are ailing do we begin to think about them. Of course,
they are absolutely necessary to us, and we should die instantly if one
of the more important of them were to stop working, but the part they
play is not one which enters actively into our consciousness, as do the
muscles, nerves, and sense organs.
Naturally, we will ask what all these other organs are for if they do
not share in our conscious life. Why can we not get along with just
those that we use for getting food, for avoiding harm, for play, and for
the other activities which they carry on? The answer to this question is
found in the fact emphasized above that continuous power development is
necessary to continued life. By themselves the muscles, nerves, and
sense organs cannot carry on power development; they require the aid of
a great many other organs to do this. Just how these other organs work
will be described later; at present it will be enough to recall that
every muscle, every nerve, and every sense organ is actually made up of
a great many of the tiny construction units--the cells about which we
were talking a few pages back--and that every one of these cells must be
developing power all the time if it is to go on living. In order to be
able to do this they must, every one, be able to oxidize fuel
continuously, and this means that they must receive constant supplies,
both of the fuel itself and of the oxygen with which it combines. Some
system of delivering these materials must be in operation, and in case
the materials have to be prepared for use beforehand this must be
provided for also. The heart, the lungs, the stomach, and the various
other organs that are useful but not conspicuous, are concerned in these
necessary jobs. In an automobile factory we have a similar situation;
the men that stand at the machines actually make the parts that go into
the finished automobiles, but unless other men are hard at work
preparing the castings, and bringing them to where the machine operators
can get them, not many automobiles will be turned out. So in the body,
unless the various organs are carrying on their work of preparing and
delivering materials to the muscles, the nerves, and the sense organs,
these latter cannot perform their tasks of getting the food for the
whole body and of securing the body against harm, nor can they carry on
the pleasant, but not absolutely necessary, activities of play and
recreation.
CHAPTER III
THE SOURCES OF FOOD
We have talked about the necessity of power development in all living
things, and have seen that power development depends on the oxidation of
fuel. Of course, our fuel is the food that we eat. No substance is
suitable for fuel unless it contains power which can be gotten out by
oxidation, and unless, in addition, it is suited to the particular kind
of oxidation that goes on in the body, and can be handled by the body.
Wood is excellent fuel for some purposes, but as a food for man it has
no value, even when ground fine and mixed with flour as was done in some
European countries during the Great War, because wood cannot be handled
by the body in the way in which a usable fuel must be. Although wood is
not good food, closely related materials are, and in fact make up the
bulk of it. All fuel food is either vegetable or animal. All animal food
traces back finally to the vegetable world, and it is an interesting
fact that we do not usually care to eat flesh that is more than one
remove from the vegetable kingdom. Animals that are flesh eaters are not
considered fit for food, except in the case of fish and birds, and the
flesh that these eat is not commonly thought of as being such, since it
consists mostly of the flesh of insects, frogs, and fish themselves.
The real sources of food, then, are in the vegetable world. Of the
countless thousands of kinds of plants that exist a few dozen have
proven to be of enough use for human food, or for food for the animals
on which human beings feed, to justify us in taking the trouble to raise
them on our farms and in our gardens. There must be something about
these particular plants to make us prefer them. If we look into the
reason for the preference we shall understand something of the qualities
which make plants good for food in the first place. At the beginning of
the chapter were set down the things which make a substance fit for
food. These are: the ability to yield power by oxidation, and a
composition suitable to be used by the body. The ability to yield power
involves the possession of a store of it. Power, or energy, which means
the same thing in our present use of the words, is never present
anywhere except as the result of an earlier exhibition of power. It is
not made out of nothing. The sun is a reservoir of energy on which the
earth draws, and we do not know with any certainty from whence the sun
got its power. The heated center of the earth itself is a reservoir of
power on which we may draw at some time in the future, when cheaper
sources are used up. Except for energy from these sources and for
trifling amounts that may be brought in by meteorites, there is none on
the earth’s surface that has not always been here. On the other hand,
the earth is constantly losing energy into space. The amount that
reaches us from the sun balances our losses into space, so that the
total energy present holds fairly steady. The energy that comes to us
from the sun is chiefly in the two forms of heat and light. In actual
horsepower the heat far outweighs the light, but in importance to
mankind one stands about on a par with the other, for while without the
sun’s heat the earth would become so cold that we would all die, without
its light there would be no food and we would all starve. This is
another way of saying that the energy that plants store up, and that we
get when we eat them, comes originally from the light of the sun.
Plants, like animals, are made up of cells. Those with which we are
familiar consist of a great many cells, of a good many different kinds.
Some are in the roots, others in the stems, still others in the leaves;
the blossoms, fruits, and seeds are made up, likewise, of cells. The
cells near the surface of the leaves, and, in many kinds of plants, near
the surface of the stems as well, contain a green substance known as
chlorophyll. This substance enables the cells in which it is present,
although we do not know just how, to manufacture sugar, utilizing the
energy of the sunlight for the purpose.
Sugar is composed of three very common chemical elements, carbon,
hydrogen, and oxygen. As we all know, hydrogen and oxygen in combination
of two atoms of hydrogen to one of oxygen form water; the most familiar
of chemical symbols is that expressing this combination, namely H{2}O.
Carbon, which we know in an almost pure state in anthracite coal, and in
even purer form in diamonds, forms a combination with oxygen known as
carbon dioxide. This is a gas; it makes up a small fraction of the air.
The amount in the air is increased whenever coal or any other
carbon-containing material is burned, since carbon dioxide is the
product of the oxidation of carbon. Except in the arid regions of the
earth there is always some water in the soil a greater or less distance
below the surface of the ground. Water and carbon dioxide between them
contain all the elements of which sugar is composed. The
chlorophyll-containing cells are the factory; the sunlight is the power;
and the carbon dioxide and water are the raw materials. Sugar is the
finished product, and wherever sunlight is falling on green plants,
whether directly or through a layer of cloud, its manufacture is going
on. Sugar will oxidize readily, and in so doing will yield abundant
power. The energy which it contains was derived by transformation from
the energy of the sunlight. With the exception of a few kinds of
bacteria every living thing on the earth depends for its food, and so
for its energy, either directly or indirectly on the sugar which green
plants manufacture. Since sugar dissolves in water it cannot easily be
held in storage, so by a simple chemical process the plant changes it to
starch, and it is in this latter form that we get it, except in the case
of a few plants, like sugar cane.
The green parts of plants are the only places where sugar is made. We
eat a certain amount of green food in lettuce and asparagus and similar
vegetables, but for the most part the sugar or starch we eat comes from
parts of plants that are not green. There is evidently a transportation
from the point of manufacture to points of storage. The means of
transport is in the sap; since starch is not soluble in water, it must
be changed back into sugar. This is done, and then, by the movement of
sap the dissolved sugar is carried to the points of storage, roots in
such vegetables as beets, underground stems in potatoes, above ground
stems in sugar cane, fruits or seeds in orchard and grain crops. In such
of these as are sweet, the sugar itself is held in storage; in most
kinds it is changed back into starch. Where the storage is in the form
of starch the vegetable ordinarily keeps better than when sugar is the
substance on deposit.
A few kinds of plants--olives, peanuts, and cocoanuts, for
example--convert the sugar into oil and store their surplus material in
that form. The chemical elements in oils and fats are the same as in
starch and sugar, although the proportions are not the same. Weight for
weight oil has more than twice the energy value of sugar; in making a
given amount of peanut oil the peanut vine used up more than twice the
amount of starch or sugar; but since energy value is what counts rather
than bulk the plant is just as well off, and perhaps better on account
of the smaller bulk occupied by the stored material. One of the very
interesting examples of oil storage is found in the very tiny plants,
called diatoms, which abound in the water of the ocean. Each tiny diatom
stores within itself an even more tiny drop of oil. Although by
themselves single oil drops would make no impression, if enough of them
could be brought into one place a respectable accumulation of oil would
result. This is precisely what the geologists tell us has happened in
past ages; the bodies of diatoms have accumulated through thousands of
years, and finally the oil accumulations have been covered over with
sediment of one kind or another. When we tap through the sediment we
strike into the “oil sand,” which contains this residue of the diatoms,
and an oil well results.
Since we depend for our food, and so for our life, on the sugar-making
activities of green plants, it will be worth our while to think for a
moment of the slowness with which the process goes on. The slice of
bread which we may eat in a dozen bites represents the result of a
season’s growth of several wheat plants, every one of which was
absorbing the sun’s energy and laying up starch grains during every
daylight minute throughout the growing season. From the standpoint of
the plant which does the storing the material which serves us as food is
the excess over the plant’s own daily needs. In most cases it would be
utilized at the beginning of the next season’s growth before the plant
had put out a leaf system, if the course of events were not disturbed to
satisfy the needs of man.
In addition to starch, sugar, and fat there is another kind of food
material manufactured by plants, known as protein. This substance is
much more complex chemically than any of the others; it contains, in
addition to the three chemical elements--carbon, oxygen, and
hydrogen--that are found in them, the element nitrogen, and usually some
phosphorus and sulphur. These materials are dissolved in the soil water
in the form of simple chemical substances, and are taken up by the plant
along with the water which enters the roots and flows as sap up to the
leaves. The same cells of the plant that make sugar have the power to
make protein, using as raw materials some of the sugar along with the
substances brought in with the soil water. The energy for the
manufacture of protein comes from the oxidation of some of the sugar or
starch in the leaf. The finished protein has about the same energy
value, weight for weight, as has the starch from which it was mainly
derived.
When an animal eats a plant or part of one, he is eating for the sake of
the sugar or sugar products which the plant has made. There is one sugar
product that is useful as food for many animals, but not for man,
except possibly to a very slight extent. That is the woody substance,
cellulose, which is formed in plants mainly as a support to the delicate
living protoplasm. Cotton fiber is nearly pure cellulose. Cellulose is
very similar to starch chemically, and is an excellent fuel wherever it
can be burned. The human digestive tract is unable to handle it in a
manner to make it usable, although grazing animals do so quite
efficiently. A good many plants make products that are either
disagreeable in flavor or actually poisonous. Of course, in such cases
the plants become useless as food unless a treatment can be devised that
will remove the objectionable material or convert it into something
harmless. The few dozen kinds of plants that we raise for food are those
that are free from harmful substances and that yield large quantities of
stored food materials, or in some cases that taste especially good, even
though they may not have much food value. Tomatoes, lettuce, and the
like, come in this latter class. The world has been pretty well
ransacked for food grains, fruits, and herbs, but probably there are
others yet to be found besides those we now have.
CHAPTER IV
THE USES OF FOOD
We have had a good deal to say thus far about power development in
living animals, and have talked about food in connection with its use as
fuel for the purpose. While we are on the topic it may be as well to say
something about other uses to which food is put in animals besides that
of serving as fuel, and also something about what is done with the power
that is developed by the burning of such food as is used for fuel. To
begin with, it is evident that one use that is made of food is to build
the body itself. The new-born infant usually weighs somewhere between 5
and 12 pounds. From birth until the body gets its growth there is an
almost continuous gain in weight until a total which may range anywhere
between 90 and 250 pounds is reached. Of course, every bit of this
additional material came into the body in the form of food. The whole
mass of the body divides itself, as has been said before, into living
protoplasm and nonliving substance. We do not know accurately what
proportion of the whole weight is made up by protoplasm; it has been
estimated at about 60 per cent, but any estimate can be only very rough
because about half of the nonliving substance consists of fat deposits
which vary greatly in different people.
In any case, that part of the food which goes to make gain in weight is
passed over to the living
[Illustration: DIAGRAM SHOWING THE RATE OF GROWTH IN MAN]
cells. If we accept the rough estimate given above, about 60 per cent is
then used for the actual manufacture of new protoplasm; the remainder is
worked over by cells specially devoted for the purpose and put into
place to serve as supporting structure, or to be held in reserve as fat.
Living protoplasm is chemically a very complex mixture. In consistency
it resembles a rather thin, transparent jelly; the thickness of the
jelly depends on how much water it contains and this varies greatly in
different kinds of protoplasm. The watery part of the protoplasm has
dissolved in it several substances; among them may be mentioned ordinary
table salt; also salts of potash and lime. Only tiny amounts of these
are present, but it is a curious fact that without these tiny amounts of
salts protoplasm cannot live. The chief solid substance in protoplasm is
protein; this
[Illustration: A FACTORY’S COOPERATION IN AIDING PHYSICAL FITNESS--THE
UNDERWEIGHT EMPLOYEES ARE GIVEN A MILK LUNCH]
[Illustration: Photo, Paul Thompson
THE WEIGHT TEST IN A CHILD’S PHYSICAL EXAMINATION TO DETERMINE HOW
NEARLY IT APPROACHES CORRECT STANDARDS OF PHYSICAL DEVELOPMENT]
material, which is one of the most complex substances known to
chemistry, has certain peculiarities which seem to fit it specially to
serve as the chemical basis of life. Evidently of all the foodstuffs
protein is the most important for the manufacture of new protoplasm, in
other words for growth. In the case of a tiny one-celled animal, whose
body is made up of protoplasm, not much else would be needed, but any
animal that has a bony skeleton has to build this up to keep pace with
the growth of the soft parts of the body. For this purpose mineral
substances are needed, chiefly lime salts.
In addition to these foods which are actually used for making new body
substance it has recently been discovered that proper growth in the
higher animals, including man, depends on the presence in the diet of
certain dietary accessories, whose use is not at all understood,
although there is no doubt of their importance. These materials, to
which has been given the rather cumbersome name of “growth-promoting
vitamines,” are found dissolved in certain food fats. Apparently they
are insoluble in water and soluble in oil. Most animal fats appear to
contain them in small amounts, while most vegetable fats do not. Milk
and eggs, which are growth foods in an especial sense, are richer in
these accessories than any other articles of the diet. The discovery of
these facts has emphasized the importance of including animal fats in
the diet of growing children, milk and eggs particularly. Since milk is
also rich in the lime salts which are necessary for bone formation it
forms the best single foodstuff for children that there is. When very
young children have to be fed on cow’s milk, which differs somewhat in
proportion from mother’s milk, it is often found necessary to feed the
top milk diluted with water, instead of the whole milk. When this is
done, lime water is usually used in part for diluting the milk, instead
of all ordinary water. In this way the proportion of lime is brought up
enough to insure that the child will get plenty of it.
In addition to the use of protein as a growth food it has another use
which no other kind of foodstuff can share. This is also because protein
is the foundation material of living protoplasm. We do not know a great
deal about what goes on in living protoplasm to make up what we call the
life processes, but we do know that these processes are of a chemical
nature, and that in connection with them there is a steady wastage of
protein. The protein that thus goes to waste is broken down into simpler
chemical compounds which are expelled from the cells. Why this occurs we
do not know, but since it does it is evident that unless the wastage is
made good the time will presently come when so much protein will have
been lost from the protoplasm that it can no longer exist as such and
must die. As a matter of fact, one might go on a diet excessively rich
in starchy foods and fats and still starve to death if there were no
protein present. This use of protein is called cell maintenance to
distinguish it from the other special use of protein in cell growth.
Evidently, whatever may be missing from the diet, protein must not be
left out. Fortunately most of our common foods contain protein. It is
especially abundant in lean meat, in dried beans and peas, and in grain.
Potatoes and most garden vegetables are deficient in protein, as are
almost all common fruits. Bread and meat are our chief stand-bys as
furnishers of protein.
Just as there are vitamines that are important for growth, so are there
vitamines that are necessary for cell maintenance. Many years ago Dr.
Sylvester Graham made himself prominent by arguing that the outer coats
of wheat grains contain something that is needed in the diet, which is
removed in the process of manufacturing white flour. He accordingly
invented a form of flour, familiar to us all under his name, which
includes some of the bran from the outer layers of the wheat. This idea,
which originated with Dr. Graham, has since been substantiated, although
not precisely as Graham had it. We know that there are necessary
accessories to the diet, but we know, also, that they are much more
widespread than Graham thought. They occur in so many kinds of
foodstuffs that anyone who eats a mixed diet usually gets enough of them
for his needs. The ill effects of their lack are most evident when the
diet is restricted to a few kinds of food which happen not to contain
them. A striking example of bodily injury directly due to the absence of
these vitamines from the food is seen among Orientals whose diet is apt
to be made up of rice plus small amounts of other substances. Of recent
years the natives of Japan and China and the Philippines have suffered
much from a disease of the nerves known as beriberi. Investigation has
shown that this disease is due to the absence from the diet of needed
vitamines, and dates from the time when rice-milling machinery was
introduced. The old hand methods of milling rice were so imperfect that
much of the hull was left clinging to the grains, but machinery
polishes the rice clean of every trace of hull. The hulls of rice
contain the accessory that is wanting from the polished grains. Wherever
it has been possible to bring about the use of unpolished (brown) rice
instead of the usual polished kind, beriberi has disappeared. Or the
same result can be secured by adding small amounts of beans to the diet.
It is probable, also, that the hulls of most grains, including wheat,
contain some of the same, or a similar accessory, so to that extent Dr.
Graham was right in emphasizing the importance of adding hulls to the
flour. Quite recently it has been shown that raw foods are richer in
these accessories than cooked, and that ordinary compressed yeast
contains more of them than any other easily obtainable material. Many
people are being benefited by taking part or all of a yeast cake daily
in a glass of milk.
For growth, or the making of new protoplasm, and for maintenance, or the
repair of protoplasmic wastage, then, we must eat protein-containing
foods, also foods containing various kinds of salts, and foods
containing the necessary vitamines. All these are to provide required
materials; the actual substances built into the protoplasm. There
remains the requirement of power, for both growth and maintenance
represent chemical activity on the part of the cell, and this activity
depends on power just as does any other activity. In saying this we are
merely saying over again in different words what was set down at the
very beginning of the book as the chief sign of life, namely, the
necessity on the part of living cells of continuous power development.
The use of food as a source of energy or power has been talked about
already, but it is necessary to say something about the different sorts
of power development that may go on in cells, and since we shall have
to talk about this a good deal, right here is a good place to bring in
for the first time a word that has come to be used whenever the matter
of the chemical activities of living cells is being mentioned. The word
is _metabolism_; when we speak of cell metabolism we mean the chemical
processes that are going on in the cells. Hereafter, instead of saying
power development, the word metabolism will be used as meaning
practically the same thing.
First of all, in describing the various kinds of metabolism that cells
may show, we have the metabolism of rest. By this we mean the power
development that is going on when the cell is doing nothing more than
keeping alive; neither growing nor showing any special activity. This is
evidently the minimum amount that any cell can show, so it is often
referred to as the _basic metabolism_. We know of at least two things
that may change the amount of basic metabolism; the first of these is a
change in temperature; when a cell is cold, its basic metabolism is less
than when it is warm. There is a very simple chemical reason for this,
namely, that chemical processes as a rule go on more slowly the lower
the temperature. Since all metabolism consists of chemical processes,
this rule applies not only to basic metabolism, but to all other kinds
as well, and, as we shall see, explains why the lower animals show such
marked differences in behavior in cold and warm weather. The second
thing that influences the amount of basic metabolism is the percentage
of water in the protoplasm of the cell. Highly organized animals, like
ourselves, are destroyed if the cells lose more than a small fraction
of their water, but there are many of the lower animals that can be
dried until their bodies contain only a very little water and still
live. This applies to microscopic forms that live in puddles and similar
places; when the puddle dries up the animal dries up too, until all that
is left of it is a tiny particle of highly concentrated protoplasm. But
this tiny particle preserves all the original cells, or at least enough
of them to make a fresh start, and a very sluggish metabolism goes on in
each cell. Of course, the advantage of this is that the stored food
materials will not be used up as rapidly as they would if metabolism
went on at the usual rate, and so there is a better chance that the
animal may survive until more water falls or drains into the puddle, or
until the particle of dust which the animal has become may be blown by
the wind where it will fall into another one. Whenever either of these
things happens the protoplasm takes up water again and the former rate
of metabolism is resumed. It is only by means of this reduction in rate
of metabolism that many kinds of animals are able to persist, for in
large parts of the globe there is a period of each year when conditions
become so unfavorable that the usual rate of metabolism could not
possibly be maintained.
Next in order to basic metabolism comes the metabolism of growth, by
which we mean the energy necessary for the making of new protoplasm. Not
a great deal is known about growth metabolism; in fact, about the only
reason for believing that it requires any energy at all is that the
metabolism of young animals, whenever it has been studied, has been
found to be greater in proportion than that of animals that are fully
grown. It is hard to account for this, unless the growth process
itself, namely, the making of new protoplasm, requires energy. When we
think of the extreme complexity of living protoplasm, we can easily
believe that its formation involves the expenditure of energy, perhaps
in considerable amounts.
The last kind of power development to be considered is the metabolism of
special activity. Most kinds of cells, particularly in highly organized
animals, have some special kind of work to do. For example, the muscle
cells have the task of making the motions; the gland cells of
manufacturing the secretions, and so on. These we speak of as the
particular functions of the cells, and the metabolism by which they are
performed as _functional_ metabolism. In some of the lower animals one
can scarcely tell where basic metabolism leaves off and functional
begins. There is a small shrimp, about a half inch long, that is found
quite commonly in small ponds. This little animal has several pairs of
legs by which he swims about, and the strokes of these legs go on
continuously, day and night, with almost no interruptions, at the rate
of a hundred or more a minute, for days or even weeks. It looks as
though this, and other animals, that are continuously on the move, were
organized without any sharp line between basic and functional
metabolism; their protoplasm liberates energy by the oxidation of food,
and various things happen as the result; among them are the maintenance
of the protoplasm and the making of motions. In the higher animals the
distinction between basic and functional metabolism is sharp, and,
necessarily so, for the well-being of any of the higher animals requires
that he shall have pretty complete control over the activities of his
protoplasm, and this he could not have if the functional metabolism were
blended in with the basic. In other words, it is as important for bodily
well-being that the cells be able to become inactive as that they be
capable of activity.
CHAPTER V
BODY CELLS
A good deal has been said thus far about living cells without anything
at all having been said to tell what they look like, or how they are
made up, beyond the statement that they consist of living protoplasm,
which is of a jellylike consistency. To look at living cells through a
microscope would almost surely be a disappointment at first, for
protoplasm is so transparent that not much of its form can be seen on
direct inspection. Fortunately for our knowledge of how cells are made
up, protoplasm that has been properly killed and preserved takes stain
very well, and different chemical substances in the protoplasm stain
differently. Thus features that could not be made out at all in the
living cells become clearly visible after killing and staining. The
first thing that attracts the attention when cells thus prepared are
studied is that every cell has somewhere within it, and usually near its
middle, a spot which is more deeply stained than any other part of the
cell. This indicates the presence of a substance or substances that take
stain more readily than the mass of the protoplasm. This peculiarity led
to the naming of the deeply staining portion of the protoplasm
_chromatin_, referring to the ease of staining. The part of the cell
which contains chromatin is called the _nucleus_. In many kinds of cells
the nucleus can be made out by an expert observer without resorting to
stains, although the details of structure cannot be seen in that way.
[Illustration: NUCLEUS OF A CELL, SHOWING CHROMATIN NETWORK
(After Martin’s “Human Body”)]
We now know that the nucleus, or rather the chromatin that it contains,
plays a remarkable and interesting rôle in the life of the cell. To this
we shall return presently. The remainder of the protoplasm, outside of
the nucleus, shows the greatest possible variety of form, according to
the kind of cell at which we happen to be looking. In some of the
simpler types this part of the protoplasm seems to be merely a nearly
uniform mass, perhaps with tiny particles scattered through it. In other
types the protoplasm is drawn out into long slender threads, and these
threads may have many branches; or the protoplasm may be distorted into
a thin shell inclosing a mass of fat; or it may be subdivided into dense
and thin portions with sharp lines of division between them. These
various forms are related to the special functions which the cells have,
and we shall learn more about them as we take up the different functions
in order. On the whole, study of cell structure shows clearly that the
protoplasm outside the nucleus carries on the greater part of the
metabolism or power development, and is correspondingly important as the
seat of the special functions shown by the cell. If it is a muscle
cell, this is the part that does the moving; if a gland cell, this is
the part that secretes. Nevertheless, the nucleus is a vital part of the
cell. It has been definitely proven that a cell from which the nucleus
is lost cannot survive more than a brief time. To gain some idea of the
actual part played by the nucleus, we shall have to return to it in some
detail.
[Illustration: DIAGRAM SHOWING CELL SUBDIVISION
_A_, a cell; _B_ to _F_, successive stages in its subdivision; _a_,
cell-sac; _b_, cell contents; _c_, nucleus. (From Martin’s “Human
Body”)]
Before undertaking a further description of the nucleus itself, we shall
be helped to an understanding of its function if we trace briefly the
history of the cells which make up our body. At the beginning, as we
probably all know, we start life as a single cell. This cell, after a
series of events which will be described in a later chapter, begins the
process known as development. Development consists of a series of
subdivisions of cell material. At first the single cell divides into
two; each of these then divides, giving four. At the next stage eight
are formed, then sixteen and so on, until finally the millions of cells
that make up the body are produced, all derived from the original single
cell. We know that in the adult body there are very many different kinds
of cells. Since they are all derived from a single cell, these
differences must have put in their appearance during the course of the
various cell divisions. In fact, this happens all along; at definite
points in the process the two cells that come from the subdivision of
some particular one will not be alike. The special kinds of cells that
are thus produced become the starting points for whole masses of similar
cells in the fully developed body. In human beings, and probably in most
other kinds of animals, the very first subdivision does not result in
any difference between the cells. The proof of this is that sometimes,
in fact fairly often, the two cells become separated. When this happens
twinning results, and the twins are exactly alike, being known as
“identical twins.” Not only are they alike in all other respects, but
they are always of the same sex, a fact that has escaped the attention
of some writers of fiction, who have made twins, identical in all other
features, brother and sister, instead of both boys or both girls. Twins
that are not identical come from different original cells that happened
to start developing together. Such twins need have no more resemblance
than any members of the same family, and may or may not be of the same
sex.
In every cell division the first step consists in a division of the
chromatin of the nucleus, which is followed by a division of the rest of
the protoplasm. The process by which the chromatin is subdivided is so
curious as to be worth a brief description. The
[Illustration: DIAGRAM ILLUSTRATING DIVISION CHANGES IN A CELL NUCLEUS
WITH FOUR CHROMOSOMES
(From Martin’s “Human Body”)]
chromatin material is not a simple lump in the nucleus. It looks rather
like a tiny string of beads thrown down carelessly, so as to become all
mixed together. Each bead is a single bit of chromatin, and these bits
are strung on a tiny thread. In an ordinary cell the beads are so mixed
together that no order can be distinguished among them, but if a cell
that is about to begin dividing is looked at it is found that the string
has straightened itself out, and also that it has broken into pieces.
The individual pieces are called _chromosomes_ and their number is
always the same for any one kind of animal or plant. There is a
parasitic worm whose cells have only four chromosomes, and the number
ranges from this up to as many as forty-eight in human beings. It may be
that other species have even more, but they become so hard to count when
there are as many as forty-eight that the number cannot be stated with
certainty. So far as can be judged, the number of chromosomes has little
to do with the complexity of the animal or plant, for some complex forms
have few chromosomes, and some simple forms many.
At the same time as the chromatin is breaking up into chromosomes two
tiny spots put in their appearance in the protoplasm of the cell on
opposite sides of the nucleus, and tiny threads extend from one spot to
the other through the nucleus. There are as many threads as there are
chromosomes, the whole group making up a spindle-shaped figure. The
chromosomes now become arranged at the middle of the spindle, and
apparently each chromosome becomes fastened to a thread. Next each
chromosome splits lengthwise through the middle and by what looks like a
shortening of the threads the split halves are pulled apart and drawn
to opposite tips of the spindle. The purpose of this elaborate scheme
seems to be to insure an exactly equal division of the chromosomes
between the cells, and the necessity of such an equal division will
become clear when we learn something of what the chromatin is for.
Meanwhile the description of cell division can be finished by saying
that after the halves of the chromosomes are pulled apart the whole mass
of protoplasm divides through the middle. As we stated above, sometimes
the cells thus produced are alike and sometimes they are different,
according to whether they are destined to become parts of similar or of
different structures. In either case the chromatin material that goes
into the two cells is exactly alike, so that if the cells themselves
become different there must have developed a difference in the
protoplasm at the two ends of the cell from which they came. Our bodies
are made up of millions of cells, of a great many different kinds, but
however different they may be the chromatin of each exactly duplicates
that of every other one, or did when the cells were first formed; there
is reason to believe that the chromatin may become changed during the
lifetime of the cells, at least in some cases.
We may be interested in inquiring how long this process of cell division
keeps up. Many children do not get through growing until they are twenty
years old or more. Does cell division keep on during all this time? More
than that; are there any cases of cell division that continue after full
growth is reached? The answer to both these questions can be given in a
brief paragraph. There are some tissues, particularly the outer layer of
the skin, the connective tissues, the blood-corpuscle-forming tissues,
and the reproductive tissues, in which cell division continues during
all or most of life. The others finish at birth or shortly thereafter.
We are born with the precise number of muscle cells with which we shall
die, unless accident deprives us of some meanwhile; and if this happens
no new ones will be formed to replace those that are lost. The same is
true of gland cells. The last cell divisions among nerve cells are
believed to occur within a few months after birth. As most of us have
observed in our own cases, bodily injuries, if at all severe, are
followed by the formation of scars. This means that connective tissue
has grown in to fill the place of the cells destroyed by the injury,
which cannot be replaced by cells of their own sort, since they have
lost the power of cell division.
We have tried, in the above paragraphs, to get some idea of what living
cells are like, and how they are derived, but have not attempted any
detailed picture of particular kinds of cells. That will have to wait
till we reach the story of the different kinds of bodily activity, when
the cells that carry on each kind will have to be described more
exactly. Something has also been told of the chromosomes, but the full
account of them and their meaning is to be taken up in a later chapter,
devoted to the matter of heredity and reproduction. In what remains of
the present chapter we wish to talk about the conditions in which cells
live so that we shall easily picture how they carry on their metabolism.
As an introduction to this topic a word may be said about the wide
differences of complexity that are found in animals. They range from the
simplest imaginable, a single cell with its nucleus and with protoplasm
that appears almost uniform throughout, to a highly organized body like
that of man, composed of millions of cells of many different kinds.
Between these extremes almost every possible form is seen. The
one-celled animals themselves show a wide range of complexity, and as
soon as animals begin to be formed of numbers of cells grouped together
the possibilities of complexity increase in proportion. One important
difference between one-celled and many-celled animals needs to be
emphasized; that is the matter of size. There are definite limits to the
size that a single cell may attain; these limits are just over the
boundary of naked eye vision. If animals are to attain larger sizes,
they must necessarily be composed of many cells. The life of a
single-celled animal presents no special problem, since it has only to
take in through its outer layer from the surrounding water the various
food materials and the oxygen which its metabolism requires, and to
discharge into the same water any chemical products that may result from
that same metabolism, and the question of whether it will live or die
depends only on whether the water in which it happens to be contains
sufficient materials and is otherwise suitable as a place to live. A
many-celled animal, whose cells are arranged in not more than two
layers, is in practically the same situation, for every cell has a
frontage on the water and so can carry on interchanges of material
directly; but the moment complexity reaches a stage where any cells are
buried beneath other cells some special arrangement must be provided so
that the buried cells can obtain the needed substances for their
metabolism. The arrangement consists, in general, of furnishing what
may be called an internal water frontage for the buried cells. In other
words, complex animals have spaces all through their bodies, and these
spaces are filled with fluid. There are no living tissues so dense that
the cells of which they are composed are completely cut off from contact
with body fluid. In thinking of our own bodies we should realize that
this same arrangement applies; every one of our millions of living cells
has contact with the fluid with which all the spaces of our bodies are
filled, and it is from this fluid that the cells obtain the materials
for their metabolism, and into this same fluid they discharge whatever
substances their metabolism may produce.
The total amount of body fluid is not large, for the spaces among the
cells are in most cases extremely tiny; it follows that with all the
millions of cells absorbing food materials and oxygen from this fluid
and discharging waste materials into it the time will soon come when no
more food or oxygen will be left to be absorbed and there will be no
more capacity for holding waste substances. If this state of affairs
were actually to happen, metabolism would come to an end and death would
be the result; evidently there must be some means of keeping the body
fluids constantly renewed in respect to the things which the cells need
for their metabolism, and constantly drained of the waste substances
which the cells pour out. The way in which this renewal is accomplished
is simple; part of the body fluid is separated off from the rest in a
system of pipes, known to us as the blood vessels, and this part is kept
in motion; at intervals along the system are stations at which the
moving fluid can exchange substances with the fluid which actually comes
in
[Illustration: DIAGRAM SHOWING HOW THE MOVING BODY FLUID RENEWS THE
STATIONARY, AND IS ITSELF RENEWED IN LUNGS, DIGESTIVE TRACT, AND
KIDNEYS]
contact with the cells; thus the stationary fluid can obtain from the
moving fluid the materials which the cells, in turn, are constantly
withdrawing from it, and can pass on to the moving fluid the products
with which the cells are continuously charging it. All that is necessary
to complete the successful operation of the system is to have additional
stations at which the moving fluid can obtain supplies of food materials
and of oxygen, and stations where it can get rid of the wastes which it
accumulates from the stationary fluid, and there must be a pump by which
the moving fluid is kept in motion. We are familiar with the moving
fluid under the name of blood; the system of pipes in which it moves are
the blood vessels; the pump which keeps it in motion is the heart; the
various supply stations include the digestive organs, the lungs, and the
kidneys. In later chapters the operation of all these stations will be
described in detail. The present outline has been given to show in a
general way how the problem of metabolism is handled in highly organized
bodies in which the individual cells have no direct access to food or
oxygen supplies.
CHAPTER VI
THE SUPPORTING FRAMEWORK
Since protoplasm is so very soft and fragile it must be supported in all
animals and plants except the very tiniest. The nature of the supporting
framework has a great deal to do with both the form and the working of
the body, so it is desirable that we become familiar with it before
trying to go further in the examination of the living protoplasm itself.
A large heavy body like that of man requires an arrangement for support
that shall meet several conditions. In the first place there must be
strength and stiffness, combined with flexibility, so that the body as a
whole shall be firm, yet not rigid. The weight, also, must be kept as
small as possible. Then every single cell, and every grouping of cells
that we call an organ, must be supported in its place securely but
without hindering the free performance of its function. Not only must
the protoplasm be held in place, but on account of its fragility it has
also to be protected against injury; the vital parts require more
careful protection than those that are less immediately essential to
life. Finally, bodily motions of all sorts depend on the framework to
give purchase to the muscles, which are the actual organs of motion, and
so to make their movements effective. For support, for protection, and
for motion, then, the framework is important.
The material that does the real supporting is not, of course, alive, for
living protoplasm lacks the necessary qualities needed here. It is
manufactured and put in place, however, by living cells. They do this by
withdrawing the special materials needed from the body fluid which
surrounds them; in large part what they get from the fluid is not the
finished substance but material from which the living cells make the
finished substance. It is then passed outside their bodies and deposited
in the surrounding space. Of course this is a gradual process. Bit by
bit the structure, bone, cartilage, or connective tissue, as the case
may be, is built up by the combined activities of many cells.
Of the three kinds of supporting material mentioned above, bone is the
most familiar. No description of its appearance is necessary, for
everyone has seen it as it appears in meat animals and in poultry, and
it looks precisely the same in man. There are several things about bone,
however, that are worth describing. One is the arrangement by which the
very hard, compact material is deposited in large masses without cutting
off the cells which are doing the depositing from their contact with the
body fluid, and so destroying them and bringing their work to an end.
The way this is managed can be made out by examination of the figure,
showing the structure of bone. At the beginning the bone cells are lying
near one of the tiny blood vessels known as _capillaries_, which are the
exchange stations for material between blood and the stationary part of
the body fluid. Thus these cells are favorably located for obtaining
materials from which bone can be constructed. As they proceed with the
formation of bone they always leave tiny passages open between
themselves and the blood capillary. Finally the capillary may become
completely surrounded by bone, but all along it will be left the
passages through which fluid can make its way from the blood to where
the cells are imprisoned within the bony walls of their own
construction. The metabolism of bone cells is not on a very active
scale; the amount of bone substance that a single bone cell has to
produce in a day is only a fraction of the amount of saliva, for
instance, that a single cell of the salivary gland turns out in the same
time; so the bone cell can manage even though its supply of material has
to come to it through a few very tiny passages in the bone.
[Illustration: CROSS SECTION OF COMPACT BONE FROM THE SHAFT OF THE
HUMERUS
_A_, bone cells; _B_, blood capillaries. (From Martin’s “Human Body”)]
Another interesting feature of bone is the ease with which it can be
remodeled. We are apt to think of bone as permanent, after it has once
been formed, but as a matter of fact bone is about as subject to change
as any of the softer tissues. This is because there are in and around
the bones, in addition to the bone-forming cells, a great many cells of
different appearance which may be named bone-destroying cells. These
latter have the ability to dissolve out the hard material which the
bone-forming cells have deposited. Good examples of their work are seen
in the hollows of the long bones. We know, of course, that the bones in
a child’s leg are so much smaller than those in the leg of an adult that
they could almost be fitted into the hollows of the latter. Evidently
the bone substance has been moved bodily outward during the course of
growth. As the bone-forming cells add material to the outer surface of
the bone, the bone-destroying cells dissolve it away from the inner
surface. The same thing happens all over the body. A child’s face grows
by an increase in size of the bones. Again the inner surfaces are
dissolved away. Apparently one condition which makes the bone-destroying
cells active is constant pressure. A good example of this is seen in
what is known as a gumboil. If a tooth becomes ulcerated, gas and pus
are formed at its root, and cannot escape since this is completely
surrounded by bone. They accordingly press upon the surrounding bone,
and also upon the sensitive tissues, resulting in extreme pain. The
pressure upon the bone starts the bone-destroying cells into great
activity and in the course of a few days they will dissolve a hole right
through the bone, allowing the gas and pus to escape to the outside, and
relieving the pain.
Of recent years school authorities have had much to say about the
importance of adjusting school seats and desks so that they shall be at
the proper height for the particular children that are to occupy them.
This is because if the feet hang clear of the ground for hours at a
time, as they will if the seat is too high, or if the body must be
screwed around to enable the child to work at his desk, as happens when
the desk is too low, there is real danger that some of the bones may
become misshapen. Most of the stoop shoulders and many of the crooked
backs that we see are the result of the habitual taking of wrong
postures. Children, and adults as well, should form habits of standing
and sitting so straight that none of the bones are put under a pressure
that may tend to distort them.
After the teeth are lost the bony sockets in which they lie are
dissolved away, making the jaws much shallower than formerly, a fact
that accounts for the shortening of the distance between chin and nose
in aged people. An important result of this dissolving away of bone by
the bone-destroying cells is that the bones are kept as light as
possible, without undue sacrifice of strength.
A second kind of supporting material is cartilage. This is both softer
and more flexible than bone. It is found in places where flexibility is
more important than great strength, as in the ears, the parts of the
nose just below the bridge, the Adam’s apple and wind pipe. The chief
difference in make-up between bone and cartilage is that while in bone
about three-fourths of the nonliving substance consists of lime salts,
in cartilage there is almost none of this material, organic substances
making up the entire mass. There are no living cells in the body that
are more poorly located with respect to obtaining supplies from the body
fluids than the cartilage cells, for as these deposit the cartilage
around themselves they leave no definite passages through which fluid
may pass; the material incloses the cells completely. Although cartilage
looks as though it were altogether nonporous, there must be some degree
of sponginess present, since the cells do succeed in getting the
materials on which their life depends. Cartilage seems to be a more
primitive kind of supporting substance than bone. This is shown by the
fact that it makes up the entire skeleton in the lowest fishes, and also
by the fact that in the higher animals, including man, the bony skeleton
starts, in large part, as cartilage. In the parts in which this happens
a mass of cartilage is deposited in the place which is later to be
occupied by bone. Then at certain points the cartilage begins to be
dissolved away by cartilage-destroying cells, which are precisely like
bone-destroying cells, and the bone-forming cells come in and build up
the real bone as fast as the cartilage is removed. This process of
replacing cartilage by bone is practically completed at birth, except in
the long bones of legs and arms. These bones, which will about double in
length during the growth of the body to adult size, as well as the other
bones, which grow to some extent, retain plates of cartilage near each
end during all the growth period, and the increase in length is obtained
by a continuous formation of new cartilage, which is continuously
replaced by bone.
The third kind of supporting material is connective tissue. This is
composed of tiny threads or fibers, some of which are inelastic, others
are very elastic. The inelastic fibers are found in places where a
flexible, but unyielding support is required; the elastic fibers are
located where elasticity is particularly important. Either kind of fiber
may be grouped into sheets, or into loose networks, or into stout
cords. A good example of inelastic connective tissue in sheet form is in
the _mesentery_ which holds the organs of the abdominal cavity in place.
Just under the skin, anchoring it loosely to the underlying muscles, is
connective tissue in network form. The tendons by which most of the
muscles are attached to the bones upon which they pull are made up of
inelastic connective tissue in the form of cords. The best example of
elastic connective tissue is in the large arteries, which are just as
elastic as best quality rubber tubing. Another good example is the large
and strong elastic cord which passes along the back of the neck in
cattle and sheep, and helps to support the weight of the head.
Connective tissue fibers are deposited by living connective tissue
cells. Since connective tissue is of open and relatively loose
construction, there is no problem presented in supplying the cells with
material. The meshes among the fibers are filled with fluid, and this
fluid has ready connection, in turn, with the blood. Use is made of the
abundance of body fluid in the connective tissue spaces whenever a
subcutaneous injection is given, for what is done is to inject the
desired material into the fluid which fills the spaces in the connective
tissue just beneath the skin, trusting that it will pass from there to
the blood, which it does rather gradually, and so is distributed about
the whole body.
While we are on the topic of the supporting framework, something must be
said about the grouping of the bones into what we know as the skeleton.
Of course it is evident that the effectiveness of the bony part of the
framework depends almost altogether on the way in which the individual
bones are grouped together. If the whole skeleton were
[Illustration: THE BONY AND CARTILAGINOUS SKELETON
(From Martin’s “Human Body”)]
composed of one great bone, or of different bones anchored solidly
together, the body would be perfectly rigid; since motion is necessary
to life, flexible connections between some of the bones are absolutely
essential. Our movements are actually made by means of muscles, but
nearly all of them become effective through the motions of bones to
which the muscles are fastened. The bones are often very irregular in
shape; careful study shows that the irregularities are due either to
provision for the contact of one bone with another in the joints, a
contact that must allow in most cases for motion of one on the other, or
to provision of places to which muscles can be fastened in such a way as
to make their pull effective. It is, of course, out of the question for
us to examine the skeleton bone by bone. Figures are given of a number
of typical bones: all that we can do in addition is to mention some of
the interesting features of the skeleton.
The skeleton of the head is called the skull; its chief features are the
brain case, the eye sockets, and the parts about the nose and mouth. The
brain case is made up of eight bones firmly joined together by saw-tooth
margins to make up a roughly spherical box which holds the brain, and
protects this delicate and vitally important organ from all injury
except the most severe. There are a number of small openings out from
the brain case through which nerves pass, and one large opening below
and at the back through which the spinal cord merges into the brain. The
bones which make up the sides of the brain case are much thickened just
behind the ears. A hollow extends from each ear into the bone, and
within this hollow, securely protected from harm, is the actual organ of
hearing. There are extensions of the hollow backward which are not
occupied by any organs, and which communicate with the cavity of the
ear. These sometimes become infected from the ear, causing the condition
known as mastoiditis. Not only is this condition excruciatingly painful,
but on account of the thin layer of bone which separates it from the
brain itself it is highly dangerous. For this reason any ear trouble
should be carefully watched lest it develop into mastoid trouble.
[Illustration: A SIDE VIEW OF THE SKULL]
Of the bones that make up the eye sockets not much need be said, except
that they have a great deal to do with determining the shape of the
upper part of the face and so the appearance. There are bones within the
nostrils that are very irregular in outline. Their effect is to increase
greatly the surface over which the air that is breathed must pass,
enabling it to become both warm and moist before entering the lungs. The
jaw bones serve as receptacles for the teeth; the lower jaw, which is
the only movable bone of the head, except for the tiny bones within the
ears, has also the duty of operating as a mill in reducing the food to
suitable form for swallowing. To aid in this function the lower jaw is
hinged to the rest of the skull in such a way that it not only opens and
closes but can slide forward and back or from side to side. All these
motions are used in chewing. There are twenty-two bones altogether in
the skull, not counting the three tiny ones in each ear which will be
described later.
[Illustration: FRONT VIEW OF TRUNK AND LIMB ARCHES
_c_, collar bone; _s_, shoulder blade; _Oc_, innominate bone (From
Martin’s “Human Body”)]
[Illustration: SIDE VIEW OF THE SPINAL COLUMN
_C_ 1-7, cervical; _D_ 1-12, dorsal; _L_ 1-5, lumbar; _S_ 1, sacrum;
_Co_ 1-4, coccygeal. (From Martin’s “Human Body”)]
The body consists of trunk and limbs, and each part has its skeleton.
The skeleton of the trunk consists of the spinal column, the rib cage,
the shoulder girdle, and the hip girdle. The skeletons
[Illustration: Photo, A. N. Mirzaoff
A FRENCH METHOD OF MEASURING VERTICAL CONFORMATION, CONSIDERED IMPORTANT
FOR ATHLETES]
[Illustration: Photo, A. N. Mirzaoff
THE CURVE OF THE SPINE MEASURED AND DRAWN FOR STUDY OF AN ATHLETE’S
POSSIBILITIES]
of the limbs are all according to a single plan to be described in a
moment. The spinal column is a remarkable example of strength combined
with flexibility and elasticity. It is made up of thirty-three bones or
_vertebræ_; each of these has a disk-shaped part known as the _body_,
and these disks are placed in line as shown in the figure. Between each
disk and its neighbor is an elastic pad composed of a mixture of
cartilage and elastic connective tissue. There is a small amount of give
in each pad and this, taken along the whole length of the spinal column,
is enough to give it the great flexibility which it enjoys. During the
day the weight of the body packs these pads down hard, so that it is
said that a man may be as much as an inch shorter at night than in the
morning. Behind the disk each vertebra has an arch of bone, and beyond
and beside this arch most of them have projections. All the arches
together make up a bony canal which contains and protects the spinal
cord. The projections serve for the attachment of the ribs and the back
muscles by which the bending motions of the body are made.
The rib cage includes the breastbone and twelve pairs of ribs. It serves
two purposes: to protect the heart and lungs from injury; and to take
part in the movements of breathing. The latter function involves some
degree of motion of the rib cage. All the ribs are attached behind to
the vertebral column in a fashion that permits of a little motion up and
down. All except the last two are fastened in front, seven pairs to the
breastbone, three pairs each to the rib above it. In breathing the
breastbone and ribs are moved up and down by muscles attached to them.
The shoulder girdle is made up of the collar bones and shoulder blades.
Each collar bone is fastened at its inner end to the upper edge of the
breastbone; this is the only direct contact the shoulder girdle has with
any other part of the skeleton of the trunk; at the point where collar
bone and shoulder blade meet, there is a shallow cup into which the head
of the skeleton of the arm fits. The arrangement is favorable to great
freedom of movement of the arm. Not only is the shoulder joint very
flexible owing to the shallowness of the cup into which the arm bone
fits, but the shoulder blade itself is capable of a considerable range
of movement. This is because it is imbedded in and held in place by
muscles. If one watches a person with bare shoulders while he raises his
arms, it will be seen that the shoulder blades do not move much while
the arms are being lifted to the horizontal position, but as that point
is passed they begin to swing outward rapidly, so that when the arms are
high above the head the shoulder blades are in a quite different
position from that which they have when the arms are down.
The hip girdle consists of five bones of the vertebral column welded
firmly together to make up what is called the sacrum, and two other
large bones known as the innominate bones, each of which, in turn, is
made up of three bones tightly fused together. The innominate bones are
firmly joined to the sacrum at the back and they meet in front, also in
a firm joint. The hip girdle or _pelvis_ is rigid, suiting it to bear
the strains that come upon it on account of its position at the junction
of the legs with the trunk. At the outer side of each innominate bone is
a cup, much deeper than the corresponding cup of the shoulder girdle,
and into this fits the head of the skeleton of the leg. The arrangement
is a typical ball and socket, and has been much copied in machinery
where a flexible joint is required. In a good many people the union of
the innominate bones to the sacrum is not so firm but that it yields
somewhat when strains are put on it. Ordinary strains in these cases
produce severe backache. Heavy strains may cause an excessively painful
as well as disabling dislocation. In either case medical attention is
needed.
Each arm can be subdivided into upper arm, forearm, wrist, and hand. The
skeleton of the upper arm is a single long bone. The forearm has two
bones, one of which is hinged at the elbow to the bone of the upper arm
in a way to limit the movement to the single back and forth swing of
which the elbow is capable. The other bone of the forearm can be rolled
over the one which is fast at the elbow; this is what happens whenever
the hand is changed from the palm up to the palm down position. There
are eight bones in the wrist; these are irregular in shape, and are so
grouped as to permit of a very wide range of movement. The bones of the
hand and fingers make up five rows numbering four bones each for the
fingers and three for the thumb. The joints are all practically simple
hinges except for the one where the thumb joins the wrist, which is a
much more flexible joint; flexible enough, in fact, to allow the thumb
to be brought opposite any of the fingers. No animal except man enjoys
this degree of flexibility in the thumb, so no animal equals man in the
nicety of the grasp, particularly of small tools. When we recall how
constantly we take advantage of this property of our hands we can
realize how greatly our superiority over the lower animals has been
aided by this rather slight structural difference between our hands and
theirs.
[Illustration: THE BONES OF THE ARM
_a_, upper arm; _b_, forearm; _c_, wrist; _d_, hand
(From Martin’s “Human Body”)]
The leg subdivides along the same lines as the arm into upper leg, or
thigh, lower leg, or shin, and foot. The order of bones is, on the
whole, the same; one in the thigh; two in the lower leg. Instead of a
flexible wrist the corresponding bones of the foot are grouped into a
less flexible, but much stronger, heel and upper instep. Two of the
bones of this group are fused together into one, reducing the total
number from eight to seven. The bones of the lower instep and toes
correspond in number and arrangement to those of the hand and fingers,
but the great toe does not have superior flexibility as does the thumb.
There is one bone in the leg, the knee cap or _patella_, that does not
correspond to any bone in the arm, although it does correspond to a part
of a bone, namely, the projection, at the elbow, of the long bone of the
forearm. A feature of the skeleton of the foot that is worth a word is
the arching of the instep. This undoubtedly adds greatly to the ease of
walking. The natural position for the foot is, of course, with both the
heel and the ball of the foot on the ground. For some reason it has
become the universal custom among civilized people to raise the heel off
the ground by adding a heel to the shoe. This does not seem to make much
difference as long as the heel is not too high. In fact soldiers wearing
properly fitted heel shoes can march as far and fast as can be expected.
Excessively high heels throw the weight too much on the ball of the
foot, thus doing away with the benefits that come from the arching of
the instep. The effect on the gait is very apparent in any one who walks
in high-heeled shoes. The foot itself does not appear to be greatly
harmed by the wearing of high heels provided the shoes are otherwise
well fitting. Whether the heels are high or low, the fit of the shoe is
of utmost importance to the preservation of the feet. Crowding the feet
into shoes that are too small in any direction is a fruitful means of
bringing on foot trouble. Wearing shoes that are loose enough to allow
the foot to turn over inside the shoe is nearly as bad. If the shoes
are properly fitted in the beginning and then the heels are kept squared
up, so that the feet will always stand straight on the ground, there
will usually be little trouble with fallen arches or other foot
disturbances.
[Illustration: BONES OF THE LEG
_a_, thigh; _b_, shin; _c_, foot; _d_, knee cap (From Martin’s “Human
Body”)]
The bones are fastened together at the movable joints by stout sheets or
bands of connective tissue known as ligaments. These hold them in place
very securely and as additional support the muscles which surround
every joint help to prevent the bones from slipping out of place. At
nearly all the joints of the body the combined action of ligaments and
muscles is sufficient to guarantee the joint against dislocation; the
shoulder joint, and to a less extent the hip joint, is more likely to
suffer this accident. The reason is that in obtaining flexibility of
movement security of attachment is somewhat lessened. If the ligaments
at the shoulder were tight enough to prevent the joint from ever
becoming dislocated they would bind it to a serious degree. Most of the
ligaments are of inelastic connective tissue, but those that fasten the
separate vertebræ of the spinal column together are elastic, allowing of
the bending in every direction which makes our backs as flexible as they
are. The only movable joints which are bound by other means than
ligaments are the connections of the ribs with the breastbone. These are
of cartilage, but the movement here is so slight that the cartilage
yields enough to permit it.
This completes our account of the bony skeleton. We shall finish the
description of the supporting framework by a word about what may be
called the connective tissue skeleton. The bony skeleton serves to
support the body as a whole and to permit the muscles to do their work;
the individual organs and the cells which make them up are held in place
by sheets and bands of connective tissue. These are coarse and strong
when their purpose is to support a large and heavy organ like the
stomach; they become finer and finer as the parts to be supported become
smaller, and when the individual cells are reached the connective tissue
which surrounds them is almost inconceivably delicate. So completely
does connective tissue permeate the whole body that it has been said
that if everything else could be dissolved away, leaving only this
tissue in place, there would still remain a model of the body, complete
to the last detail.
CHAPTER VII
MOTION
Our account of the body has now reached the point where we can take up
in detail the special activities of the different kinds of cells. The
first to be considered is motion, both because it is the familiar sign
of life, as pointed out in the first chapter, and because it has so much
to do with everything that enters into life. There are probably no
animals that live out their entire lives without making any active
motions, although some parasites, like the tapeworm, are stationary most
of the time. There are a number of different ways in which movements are
brought about. The very simplest animals, which consist of nothing but a
bit of protoplasm, move by causing the protoplasm to flow bodily in one
direction or another, a projection of part of the protoplasm being
balanced by withdrawal of an equal part on the opposite side, and the
whole mass progresses in the direction of the first projection. Next
beyond this simplest means comes motion by tiny threads of protoplasm
which project beyond the surface of the cell and whip back and forth.
The stroke of these threads or _cilia_, as they are called, is stronger
in one direction than in the other, so the effect of their beating is to
propel the cell of which they are part in one direction through the
water; or if they are on a surface which is stationary they set up a
current in the water itself. This latter is the means by which oysters
and similar animals which are anchored to the rocks get their food
supplies. In some one-celled animals there are only one or two large
cilia at one end; these beat back and forth, propelling the animal much
as a fish swims.
The commonest, as well as the most effective, means of making motions is
by cells specially developed for that purpose. These are called muscle
cells, and every highly organized animal depends on them for most if not
all of the motions which take place in its body. In muscle cells the
functional metabolism takes the form of forcible changes in shape of the
cells by which bodily motions are brought about. A muscle cell might be
described as a mechanical device for transforming the chemical energy of
burning fuel into the energy of motion. We have something comparable in
the automobile cylinder, where the energy obtained from the explosive
burning of the air-gas mixture drives the piston and so propels the car.
Of course the two devices are not even remotely alike in the actual way
in which they operate; their resemblance is purely the general one of
converting one type of energy (chemical) into another type (motion).
There are three kinds of muscle cells in our bodies. The simplest are
those that are found in the wall of the stomach and intestines and other
internal organs that are capable of movements; the next kind is found
only in the heart; the third, and most complex, makes up the bulk of our
muscular tissue; it is the muscle that moves the bones. The first kind,
because it shows no particular markings when examined through the
microscope, is usually called smooth muscle; the second kind is known as
heart muscle; the third kind, because it moves the skeleton, is named
skeletal muscle. We shall devote most of our attention to this third
kind of muscle, because it is a much more efficient machine than the
others, and also because it has to do with our general bodily movements
instead of with motions of internal organs.
A single skeletal muscle cell is an exceedingly slender fiber, much
smaller than the finest thread; it may also be very short, not more than
a twenty-fifth of an inch long, or it may be as much as an inch long. A
muscle is made up of many of these fibers grouped side by side in
bundles, and also, if the muscle is long, placed end to end. The fibers
are held in place and fastened together by connective tissue. Lean meat
consists of thousands of these muscle cells with their connective tissue
fastenings. In coarse meat there is relatively more connective tissue
and less actual muscle tissue than in the finer grades. In every muscle
the connective tissue is loose enough to allow body fluid to penetrate
among the muscle cells. Blood vessels are also distributed through the
mass of the muscles between and among the cells; thus their nutrition is
provided for.
Although not all muscle cells are exactly equal in power, on the whole
the _force_ that muscle can show is the force of one cell multiplied by
the number of cells that can join in the pull. A strong muscle must have
many cells side by side; in other words it must be thick. Also, the
_distance_ through which muscles can make movements depends on their
length, so a muscle that has to pull for a considerable way must be
long, and since single muscle cells are short there will have to be a
good many cells end to end to make the whole muscle long enough for its
task. The actual make-up and arrangement of muscles in the body depends
in part, therefore, on the thickness and length needed for the
particular work to be done, and in part on the architecture of the part
of the body where the muscle is located. For example, the strongest
muscle in the body is that by which one rises on the toes. This muscle
operates by pulling upward at the back of the heel. If it were located
right at the ankle, where it would have to be if attached directly to
the place where its force is exerted, the resulting clumsiness can
easily be imagined. By shifting it up to the middle of the lower leg
room is found for the large mass of muscle needed for the work. The
connection with the heel is made by means of a long and very strong
tendon, known as the tendon of Achilles, because that was the part
Achilles’ mother failed to immerse when she was dipping the infant in
the river Styx to make him invulnerable. Other equally good examples are
the muscles for operating the fingers. If placed in the hands, the
latter would be too bulky and clumsy for any kind of efficient use. By
placing them up in the forearms out of the way, and connecting them with
the fingers by long tendons, delicacy is secured for the hands.
The muscles of the arms and legs are arranged in groups about the
joints, and these groups always include opposing sets. Thus if the joint
is a simple hinge, as at the elbow, where the only motion possible is
back and forth, there will be one muscle to bend the joint and another
opposing muscle to straighten it out again. The first is known as a
_flexor_; the second as an _extensor_. In the arm the biceps, on the
upper surface, is the flexor and the triceps, on the under side, the
extensor. Joints that permit of motion in several directions have
correspondingly more opposing sets of muscles acting upon them. The same
scheme applies to the trunk, but since in the trunk instead of a few
very movable joints we have the whole row of slightly movable vertebræ,
the grouping of muscles is more complicated. Not all the skeletal
muscles work about joints. The tongue, the muscles of the lips and about
the eyes, those along the front of the abdomen, and some others are
attached to bones only at one end or not at all, and do their work by
pulling upon one another.
[Illustration: THE BICEPS MUSCLE AND THE ARM BONES
(From Martin’s “Human Body”)]
In earlier paragraphs we have seen that the movements made by muscles
represent their functional metabolism, and also that the actions of
whole muscles are merely the sum of the actions of the individual cells.
Our present task is to see how muscles act; in other words to examine
their functional metabolism. One feature that must be in mind from the
very beginning is that the functional metabolism of muscle cells is
under control; they do not go off at random, but only when started. This
is more or less true of the functional metabolism of all the cells in
highly organized animals. The agency that starts them off is named a
stimulus. To picture how stimuli act we shall have to think for a moment
of the state of affairs in cells at rest. As we have tried to make
clear, cells at rest are not stagnating; a more or less active basic
metabolism goes on within them all the time. This metabolism is of such
a sort that it does not disturb the balance existing within the cell.
The various chemical processes go on, using up material and producing
wastes, but without arousing the additional chemical processes of
functional metabolism. Meanwhile the substances that are required for
this latter are present in the cell, so that when the disturbance that
we call a stimulus comes along there is an increase in the total amount
of metabolism, the extra chemical processes being those which perform
the special function of the cell. In the case of muscle cells the
stimulus ordinarily reaches them by way of the nervous system, although
electric shocks, sharp blows, some irritating chemicals, and perhaps one
or two other kinds of disturbance can act as stimuli. The effect of the
stimulus is to start certain chemical processes; these in turn bring
about the forcible shortening which is the thing that happens in active
muscle. In skeletal muscle the shortening may be very rapid; the muscle
can contract and relax again more quickly than the eye can follow. This
is true at the temperature of our bodies. In cold-blooded animals, like
fish or frogs, muscles become sluggish when they are cold. We see here
one of the advantages we enjoy in having bodies that stay at the same
temperature the year around; if our bodies cooled off in cold weather as
do those of frogs, we should have to do as they do, become inactive
whenever the weather becomes cold. As each muscle cell shortens it pulls
upon the connective tissue that surrounds it; this communicates with the
connective tissue of other cells, and all the connective tissue within
the mass of the muscle fastens to the very stout sheets or cords of the
same at the ends which are called tendons, by which the muscles are
attached to the bones. Thus, although the pull of any single cell is so
feeble as to be scarcely measurable, when hundreds or thousands of them
pull all at once the effect may be very powerful.
We are familiar with the very wide range of effort that our muscles can
show. They may contract with utmost delicacy, as when we hold a humming
bird’s egg in our fingers, or they may pull with a force, in our largest
muscles, of several hundred pounds. Of course this possibility of
variation is of great advantage in our use of our muscles. It depends
upon the very large number of individual fibers of which even our
smallest muscles are made up. Whenever any single fiber contracts, it
pulls to its full extent; if only a few become active, the pull of the
whole muscle will be slight; as more come into action, more force will
be exerted; the muscle will show its utmost power when all the fibers
are contracting at once. We are conscious of greater mental effort when
we make a powerful muscular contraction. This can be explained as due to
the greater nervous discharge required to excite all the muscle fibers
at once.
One feature of muscular action calls for an additional word. This is the
temporary loss of power, resulting from too long-continued use, which
is called _fatigue_. We know that a well-constructed machine can operate
day in and day out without having to stop to rest; why cannot our
muscles do the same? Evidently the necessity of resting cuts down the
possibilities of life more than any other one thing; our real life is
only two-thirds as long as it counts up in years because we have to
spend one-third of the time in sleep. Of course muscular fatigue is not
the only kind; there is nervous fatigue, as well, about which something
will be said later. The activity of our muscles is based on functional
metabolism; it follows, therefore, that fatigue is also due to
metabolism. We can think of two ways in which metabolism might cause
fatigue; the first of these is by using up the materials which furnish
energy; clearly no cells can go on working after they have exhausted
their supplies of fuel. The second results from the fact that metabolism
produces waste products. It is a familiar fact of chemistry that when
the substances formed in chemical processes are not removed they
interfere with the processes themselves. In active muscles very rapid
metabolism is going on and large quantities of waste substance are being
formed; these have to be discharged from the cells into the surrounding
fluid, and removed from there in turn by the blood. We can easily
imagine that this might not take place as fast as necessary to keep the
cells from becoming more or less clogged; in fact this clogging is
exactly what happens, so that muscles begin to show fatigue some time
before their supplies of fuel material are used up.
One familiar fact of muscular fatigue is that soreness, which indicates
that fatigue has really
[Illustration: Photo, Metropolitan Museum
MUSCULAR DEVELOPMENT OF AN ATHLETE--THE DISCUS THROWER OF MYRON]
[Illustration: A MODERN “VICTORY”--MISS SABIE AT PRACTICE]
been present in large amount, occurs much more often when we use our
muscles in ways to which we are not accustomed than when they are
exercised according to habit. It is the experience of every one who does
manual labor that when he gets a new job, one that calls for different
use of the muscles than he has been in the habit of, his muscles are
very sore until he is “broken in.” After that, although he does as much
or even more work than at first, he no longer becomes sore. This is
explained as being due to two things. First, whenever we make an
unaccustomed movement we overstimulate our muscles; that is, we call
more fibers into action than are necessary to do the job; as the motion
becomes familiar we cut down the action to that which just meets the
demand. Thus there is a great deal more metabolism than necessary when
unfamiliar motions are being made. Then, secondly, there is a spot in
every muscle cell, just at the point where the nerve makes its
connection with the muscle, that is more easily fatigued than any other
part of the muscle cell. This spot, by becoming fatigued first, tends to
cause metabolism to stop in time to prevent the rest of the cell from
being seriously fatigued. Only when we are so much interested in what we
are doing that we pay no attention to the fatigue of this safety spot,
or when necessity keeps us at work after we would quit if we had our own
way, do we push the metabolism so far that muscular soreness results.
Other types of fatigue, including feelings of exhaustion, are due to
effects on the nervous system, and will be described when we have that
system before us.
Before we leave the subject of the skeletal muscles it will be
interesting to say a word about the different kinds of motions that
they bring about. We have already seen that they work by pulling at the
joints, and we have no intention of enlarging on that topic. What we
want to do here is to group the bodily motions into a few classes,
regardless of what joints are actually moved. First, and most important,
comes _locomotion_; by that we mean any motions that move the body from
one place to another. Under that head we have walking, running,
swimming, jumping; in birds, flying. Next in order comes _grasping_;
this includes all motions by which we take hold of anything. We can
realize the importance of this group of movements when we think that our
fore limbs are specifically grasping organs, while in the great majority
of animals they are organs of locomotion along with the hind limbs.
Originally grasping had to do, undoubtedly, with the taking of food and
not much else. In civilized man we have in addition the use of all kinds
of tools from the coarsest to the finest. In most four-legged animals
the chief organ of grasping is the mouth. We still use our mouths to
some extent as grasping organs, and could probably learn to make even
more use of them in that direction if forced to it. _Chewing_ and
_swallowing_ make up a group of movements concerned primarily with the
handling of food after it has been grasped. Not much need be said about
them. Of small extent but great importance are the motions connected
with _sense perception_; these include chiefly the motions of the body,
neck, and eyes in _vision_; we are constantly turning to look at
something; in such animals as horses movements of the ears help greatly
in _hearing_; and both man and animals make sniffing motions to increase
the keenness of _smelling_. There is a group of motions devoted to
_voice production_ (including breathing). In man the vocal cords,
tongue, and muscles of the cheeks are the chief muscles that have to do
with the voice, not including the muscles of breathing, which, of
course, are essential. The interesting things about the vocal cords are
the excessive fineness of their operation, enabling expert singers to
produce tones that vary by only a few vibrations a second, and the
amazing exactitude of the control that the nerves have over them, so
that good singers can set them at the tension needed for producing a
particular tone with absolute certainty. The tongue is not a single
muscle, but a mass of several muscles working one upon the other. It
plays a part both in voice production and in the chewing of food. As an
organ of voice production it helps by changing the shape of the mouth
cavity. Speech depends very largely on this, since not the tension of
the vocal cords but the shape of the mouth and throat determines the
making of letters and syllables.
In addition to these familiar uses of the muscles there is a use which
is just as important but about which we are apt to think less. This is
their use in connection with posture, the taking and holding of
particular bodily positions. Posture is unlike other muscular activities
in several things. In the first place there is a steady, but rather
feeble, tension which can be held without marked fatigue for long
periods; all other forms of muscular contraction become severely
fatiguing rather quickly if held steadily. In the second place the
nervous control of posture seems to be different in some respects from
our ordinary control of our muscles. Finally there is some doubt as to
whether the contractions of the muscles themselves are the same.
Measurements of the functional metabolism of posture show that it is
much less than would be expected if the muscular action were of the
ordinary type. This, of course, explains why posture is less fatiguing
than other forms of activity.
The other two kinds of muscle, heart muscle and smooth muscle, must have
a word of description. Heart muscle contracts quickly and powerfully, as
does skeletal. It differs from skeletal in not depending on nervous
stimulation to make it contract; the heart can be cut clean out of the
body and will go on beating for a short time; in cold-blooded animals,
like frogs or turtles, for a long time. This could not be true if the
heart muscle had to be aroused to activity by nerves. Besides being
automatic, heart muscle shows the peculiarity that whenever it contracts
all the fibers join. We do not have a varying strength of pull shown by
heart muscle as we do in skeletal. As we shall see, it would be a
serious disadvantage rather than an advantage if heart muscle were to be
like skeletal in this respect.
Smooth muscle has the duty of operating the internal organs. For this
function no great strength is required; the motions do not have to be
powerful. Nor is rapid motion important. Smooth muscle does not have to
be so highly developed, then, as is skeletal. It is sluggish and rather
feeble in its actions. There are, however, two points of superiority
about smooth muscle, which fit in well with its special task. The first
of these is its freedom from fatigue. There are in the body numerous
smooth muscle masses that are in contraction practically all the time.
This would be impossible if fatigue were to develop. These masses make
up what are called the _sphincters_, rings of muscle surrounding
openings like that from the esophagus to the stomach or from the stomach
to the small intestine. It is the duty of these sphincters to hold the
openings closed all the time except occasionally when they open for just
an instant to let material through. The second point about smooth muscle
which fits it for its work is that it is capable of stretching out
greatly or contracting sharply without much difference in the force with
which it is pulling. For example, at the beginning of a meal the walls
of the stomach are drawn up, so that the food that is swallowed enters a
small space. With the progress of the meal the stomach enlarges, so that
at the end it has a much greater bulk than at the beginning. But the
actual pressure of the stomach upon its contents is about the same as at
the beginning. If the stomach were an ordinary elastic bag this could
not happen; the walls would have to stretch as the stomach filled, and
the stretching would mean greater pressure. Since the stomach walls are
of smooth muscle they adjust themselves to the progress of the meal. It
is important to note that there is a limit to this possibility of
adjustment. If one is so greedy as to keep on stuffing after the stomach
has reached its full size, stretching does occur, and if this is
repeated it may lead to a diseased condition known as “dilated stomach,”
which will cause much digestive trouble.
CHAPTER VIII
SENSATION--INTERNAL AND CONTACT SENSES
We have talked a good deal about muscles and the different sorts of
activities they can perform. We have also mentioned the fact that the
skeletal muscles are under accurate nervous control. Our next task is to
investigate the control of this nervous control; in other words to find
out just what it is that causes the nerves to stimulate the muscles so
that they shall perform as skillfully and usefully as they do. In
Chapter II we saw that our bodily movements are adjusted to our needs
through the sense organs. These bring information of the situation and
we act accordingly. We may group the kinds of information which the
sense organs furnish under three heads; first, what is going on inside
our bodies; second, what is happening at the surface of the body, and
third, what is happening at a distance from us. The senses which bring
the first kind of information are called the _internal_ senses; the
second group are the _contact_ senses; and the third are the _distance_
senses. We need to remember that the primary purpose of the senses is to
guide our muscles, and that our muscles are to find food for us, to keep
us from bodily harm, and to assist in the perpetuation of life by
propagating and caring for the young. By keeping these facts in mind we
shall have no difficulty in understanding the way in which the various
senses do their work.
Pain, hunger, and thirst are the internal senses with which we are most
familiar. Pain is evidently a protective sense. It is never aroused
unless something is amiss; for that reason pain should never be
neglected. Of course, in the majority of cases the pain is due to some
simple disturbance which can be located, and if no permanent harm is to
follow, or if no relief is possible, the heroic bearing of the pain is
meritorious; but thousands of women, thinking mistakenly that to
complain of suffering is a sign of weakness, or hoping to spare loved
ones distress, bear in secret or make light of pains that are the signs
of insidious disease, curable if taken in hand early enough, but sure to
cause acutest suffering and untimely death if allowed to go on
unchecked. Unfortunately our most dangerous internal enemies, the
organisms of infectious disease, do not give warning of their attack by
causing pain until the disease itself is so far advanced that there is
no escaping it. In this respect pain falls short of being efficient as a
means of warning us against impending injury.
Hunger and thirst are the stimuli which drive us to the taking of food
and water. It is interesting to think that of all the living things that
roam the earth only men have discovered the connection between the
taking of food and the avoidance of starvation; all other animals are
impelled to nourish themselves wholly through the operation of these
senses. There are two distinct phases to hunger. The first is appetite,
and this by itself seems not to be a sense in the strict meaning of the
word, but rather a memory of agreeable experiences associated with the
taking of food. In man appetite is often sufficient by itself to lead to
eating, as is proved by the frequency with which food is eaten between
meals when there cannot possibly be any genuine hunger, but probably in
animals it acts to arouse genuine hunger, rather than to cause eating by
itself. Genuine hunger is a sense as definite as any other. It is
aroused by spasmodic contractions of the stomach. These contractions
cannot occur except when the wall of the stomach is in a certain state
of tension. Various things can influence the coming on of this degree of
tension in the stomach, and so the possibility of hunger. Appetite
itself probably does this very effectively. Habit seems also to have
something to do with it. Hunger is usually felt just as mealtime draws
near, and it is often much keener at noon or night than before
breakfast, although the stomach has been longer empty at breakfast than
at any other meal. A curious fact about hunger is that it may disappear
completely after a few days of complete starvation. Contrary to the
popular idea that hunger becomes more and more acute as starvation
continues, the testimony of practically all persons who have starved for
more than a few days is that all sensations of hunger, as well as all
strong longings for food, subside and do not return. This is especially
true if the body is kept quiet and if the mind is diverted, so that
recollection of meals particularly enjoyed shall not come up.
Thirst is due to actual drying of the throat. When the cells lining that
region become deficient in moisture the sense is aroused. The drying may
occur from without or from within. When it occurs from without, as in
sleeping with the mouth open, relief can usually be obtained by merely
swallowing saliva copiously. The same treatment helps for the moment
when the lack of moisture is due to deficiency in the amount in the
body, but in this latter case no permanent relief can be had except by
the taking of water. When the amount in the body falls below the proper
level no comfort can be had until the loss has been made good. An
interesting thing about thirst is that it is the only sense which is
said never to be lost or seriously impaired by disease.
In addition to these familiar internal senses we have some that are less
well known. They are for the purpose of what may be described as the
routine guidance of the muscles. The act of walking, as we well know, is
made up of a series of muscular movements which are both accurately
timed and accurately graded. We obtain startling realization of this
when we come to the bottom step on our way down stairs without noticing
that we have arrived there. This timing and grading are done for us by
our bodies without our having to attend to it. The amount of labor that
is saved is shown by walking upon railroad ties. These are irregularly
spaced, and on that account it is necessary for us to pay attention to
every step. There is no comparison between the fatigue of this kind of
walking and ordinary progress along a smooth path. The senses that keep
track of the position of the body and of individual muscles are known
respectively as the equilibrium sense and the muscle-and-joint sense.
The equilibrium sense has as its organ a part of the internal ear.
Deeply imbedded in the bone is a series of chambers and canals lined
with a delicate membrane and filled with liquid. The canals, which are
three in number in each ear, are semicircular in shape, and accordingly
have been named the _semicircular canals_. One of them is horizontal;
the other two are vertical, and the two vertical canals lie at right
angles to one another. This arrangement makes it inevitable that any
movement of the head, in any direction whatsoever, will register
differently on the canal system than any other movement, which is
exactly what is required to make the apparatus efficient as an organ by
which motions of the body are kept track of and guided. Along with the
semicircular canals is a structure known as the _vestibule_ which
registers the position of the head, and so indirectly of the body, when
no movements are being made. We are not ordinarily conscious of the
working of these senses; they carry on their guidance of muscular
movement without our attention. We can, however, pay attention to what
they show if we wish. For example, one who is swimming under water is
never in doubt as to whether his head is turned up or down, even though
his eyes may be shut. His knowledge of position in such a place is
obtained from his equilibrium organ, even though he may not be aware of
the fact. Sometimes the organ becomes diseased. The results, so far as
the victim is concerned, are highly distressing. He usually has to stay
in bed because he cannot balance himself well enough to get about.
The organs for muscle-and-joint sense consist of tiny spindles
distributed around the joints and embedded within the mass of the
muscles. They are arranged so as to be affected by every motion of a
joint or every contraction of a muscle. They register not only the fact
of motion but also the extent. There is a disease, commonly known as
locomotor ataxia, in which the muscle-and-joint sense is impaired or
lost, particularly in the legs. The result is that walking becomes
difficult and unsteady, and usually impossible when the eyes are shut or
the room is dark. This is because the victim learns to make his sight
serve instead of his muscle-and-joint sense for guiding his muscular
movements, and when this also is withdrawn all knowledge of where his
legs are or what they are doing fails, and the only course is to fall
down or lie down as quickly as possible.
We have some additional bodily sensations, such as nausea, repletion,
fatigue, ill feeling or _malaise_, which guide our conduct more or less,
and are not very different in consciousness from hunger or thirst. So
far as is known there are no sense organs by which these sensations are
aroused. They are not strictly senses, therefore. We do not know enough
about how they originate to say anything more about them.
The _contact senses_ are touch, warmth, cold, and taste. Pain that comes
as the result of bodily injury might also be classified as a contact
sense, since its cause is something that comes in direct contact with
the body from outside, but it differs from internal pain only in its
source and not at all in the sensations it arouses, so there is no need
of describing it over again. The sense of touch is the fundamental
sense; the very lowest animals, even those that have no specially
developed sense organs, and few organs of any kind, react to the contact
of objects with their bodies just as the highest animals react to the
sense of touch. When no other information is available, that of simple
contact guides the animal in its securing of food and its avoidance of
harm. In accordance with this primitive character of the touch sense,
the psychologists tell us that we interpret the information from our
more highly developed sense organs, sight particularly, in terms of the
feel of objects. When we look at anything our judgment of it actually
consists in an idea of how it would feel if we were to take hold of it.
Our touch organs consist of tiny spots scattered all over the surface of
the body. They are much closer together on some parts than on others.
The total number is estimated at a half million or more. A good way to
test their sensitiveness is by pressing down on different parts of the
skin with fine hairs. When this is done it is found that the most
sensitive regions--the tip of the tongue, for instance--are fifty or
sixty times as sensitive as the dullest regions, like the small of the
back. To obtain sensations of touch it is necessary that there be
unaffected points alongside those that are affected. If all are acted on
alike, there will be no more sensation than if none is acted upon. This
can be shown by dipping the hand into quicksilver. The very heavy liquid
presses on all the touch points hard enough to affect them, but since it
presses on all alike nothing at all can be felt except along the line
where the hand enters the quicksilver where the pressure is strongly
marked. It is this feature of the touch sense that makes the wearing of
clothing bearable. If we had to feel the contact of the clothes
constantly we should presently find them so trying that we could no
longer endure them. We do feel rough places and are often seriously
annoyed by them, so we can judge what would be the effect if the whole
surface were felt as plainly.
Closely related to touch is the sensation of tickling or itching.
Curious facts about this sensation are the violence of the feeling that
may be aroused by very delicate irritation, drawing a thread along the
corner of the nose, for example; the persistence of the feeling beyond
the actual irritation; and the effectiveness of scratching as a means of
alleviating the condition. Almost nothing is known in explanation of any
of these peculiarities.
In addition to organs of touch the skin contains two kinds of organs for
perceiving differences of temperature. The first of these detects
warmth; the second cold. It is by means of these senses that we judge
whether the place where we are is of a suitable temperature in which to
remain; whether we should be quiet or active; whether special
provisions, like changes in the clothing, are necessary. In the case of
both senses the temperature of the skin is the comparison point. We
judge that an object is warm or cold according as its temperature is
above or below that of the skin which touches it. The ears are usually a
few degrees cooler than the hands; thus it is possible for one and the
same object to feel cold to the hands and warm to the ears. The two
kinds of temperature organs are side by side in the skin, although there
are many more “cold” spots than “warmth” spots. Very warm objects affect
both kinds, and then we get the sensation that we call “hot,” as
distinguished from merely warm. The cold spots are a little nearer the
surface of the skin than are the warmth spots; for this reason a hot
bath may feel cold at the very instant of stepping into it, although the
sensation changes to hot almost at once. We need to remember that our
sensations of warmth or cold depend altogether on the state of the skin,
and tell us nothing at all about whether our bodies as a whole are warm
or cold. Because the blood is always warm a flushed skin always feels
warm, and to produce flushing by means of alcohol has long been used as
a means of making the body feel warm and comfortable. This may be a
serious mistake in cold weather, for to drive the blood to the surface
then may mean that the body as a whole will cool off to the point of
actual injury. It is better to feel cold and conserve the body’s heat
than to feel warm and waste it.
[Illustration: TASTE BUDS
(From Martin’s “Human Body”)]
The last of the contact senses is that of taste. This is found only on
the tongue. Scattered about on that organ are many tiny sense organs
known as taste buds. These are usually in little hollows, so they cannot
be affected unless liquids which can enter the hollows are on the
surface of the tongue. If the tongue is wiped dry and then dry sugar is
sprinkled on it, no sweet taste will develop so long as the dryness
continues. The purpose of taste is evidently to give final information
about the food after it has passed the inspection of the other senses
and has been inserted into the mouth, but before it is swallowed. In the
higher animals there has been a subdivision of this sense into two. The
other is the sense of smell. In large part smell is a distance sense,
and will be treated when we are talking about the distance senses. Smell
has monopolized most of the properties of food-judging, so there is left
for taste proper only four kinds of perception. These are sweet, sour,
salty and bitter. We have, apparently, four kinds of taste buds, one for
each of these kinds of taste. All the other sensations that we call
taste are flavors, and are really smells. Of the four tastes sour and
bitter would probably be called warning and sweet and salty
recommending. Only by practice do we come to care for bitter foods, and
children usually object just as strongly to those that are sour.
Tropical savages, for whom salt is a rarity, esteem it much more highly
than sugar, which they can usually get in abundance.
In concluding this chapter we need to remember that the contact senses
make up the court of last resort; by the time anything comes close
enough to the body to act upon any of them it is so close that the
effect in guiding the muscles must be immediate; there is no time for
deliberation; whatever the muscles are going to do in response to
information thus obtained must be done at once. Later we shall see how
this affects our whole bodily make-up.
CHAPTER IX
SENSATION--DISTANCE SENSES
The three senses that give us information of what is happening beyond
the surface of our bodies are smell, hearing, and sight. Since smell is
closely related to taste, which was talked about in the last chapter, we
shall take it up first. Smell is like taste in that it is aroused by
chemical substances, but to be smelled these must be in gaseous form,
not dissolved in water, as for taste. The organ for smell is in the
upper part of the nasal chamber. There are really two of them, one in
each nostril. They are made up simply of little patches of mucous
membrane, as the membrane that lines the nose is called, in which are
many of the particular kind of cells that are affected by odors. An
interesting thing about these patches is that they are not in the part
of the nostrils through which the main current of air sweeps in
breathing, but in a little pocket off this main channel. If air
containing an odorous substance is breathed in or out, a little of it
works its way into the side pocket and is smelled. If we wish to get
more of the odor we do it by sniffing, which is changing the shape of
the nostrils to throw the air current more directly against the smell
organs.
These organs are amazingly sensitive. It is hard to appreciate the
minuteness of the amounts of material that can be smelled. Especially is
this true of those animals that have a really keen sense of
[Illustration: Photo, Fifth Avenue Hospital
A NEW HOSPITAL DESIGNED TO SUPPLY THE MAXIMUM OF LIGHT AND AIR--CHIEF
REQUISITES FOR HEALTH--TO EACH ROOM]
[Illustration: Copyright, Fifth Avenue Hospital
THE ARCHITECT’S PLAN WAS MADE WITH THE NEED FOR LIGHT AND AIR IN MIND]
smell. The amount of substance that rubs off from a rabbit’s feet onto
the ground at each step cannot be much to begin with; yet this continues
for hours to give off gas into the air, and a dog coming along at any
time meanwhile will get enough of the gas into his nostrils to smell it.
Fortunately for our comfort the sense of smell fatigues very rapidly. An
odor that is excessively disagreeable at first presently no longer
troubles us. If it were not for this, it would be almost, if not quite,
impossible to obtain laborers in those industries where the odor is
necessarily bad. There is, however, a source of danger in this quick
subsidence of smell perception. About our only method of judging offhand
as to the ventilation of a room is by the smell, and this fails as a
guide when we have been in the room for a time. Persons coming in from
outside are often struck by the bad state of the air in rooms whose
occupants are not conscious that there is anything amiss. Because of
this the ventilation of schoolrooms usually is not, and never should be,
left merely to the judgment of the teacher, but definite rules are laid
down as to opening of ventilators or windows.
When the gas that is smelled is part of an inward air current we
recognize it as coming from the outside and call it an odor; when it is
part of an outward current we call it a flavor. On account of the rapid
fatigue of the sense of smell we are unconscious of the smell of our own
breath, but can get fresh smells from within, and these come,
practically always, from materials that have just been taken into the
mouth. In comparison with taste flavor furnishes great variety of
perception. As persons become connoisseurs in food their enjoyment
depends more and more on flavor and less and less on taste. The
sensation from spices is a combination of flavor with irritation of the
tongue that is partly pain and partly touch. The sense of smell has
evidently a two-fold use; it makes us aware that there is food in the
vicinity, or sometimes that disagreeable things are near at hand; and it
shares with taste the duty of enabling us to judge of the food as it is
being eaten. Agreeable tastes, flavors, and odors add much to the
enjoyment of life. Within reasonable limits it is well to cultivate this
kind of enjoyment, for while there is no doubt that it can be overdone,
as in the excessive lengths to which the decadent Romans went to gratify
their taste and smell, neither is there any doubt that bodily health in
general, and the bodily function of digestion in particular, benefit
definitely from the kind of enjoyment that savory food and delightful
odors bring. It is the duty of those charged with the responsibility of
preparing and serving food to take pains that full advantage is taken of
the possibilities present in what food is to be prepared. This does not
mean expensive food; what it does call for is skillful handling of all
food, whether cheap or costly.
In introducing the subject of hearing we shall have to say a few words
about that which is heard, namely sound. Any object that has any degree
of elasticity at all is apt, if struck or rubbed or otherwise set
suddenly in motion, to start vibrating back and forth; the vibrations
will nearly always be regular, and will occur at a rate that is the same
for that particular object whenever it vibrates. The rate depends on the
size, the character, and the degree of stretch of the object. Air is to
all intents and purposes perfectly elastic; it is set vibrating by any
object that is vibrating in it, but since it has no particular size nor
degree of stretch it takes the vibration rate of the object that started
it going in the first place. The vibrations once started in air spread
in all directions, just as waves spread from a stone thrown into a pond,
and when these air waves strike upon another object that is free to
vibrate they will set it going at the same rate. The human hearing
apparatus is a device which is set in vibration by air waves, and the
result is called sound. The ear is limited in its ability to respond to
vibrations; they must be neither too fast nor too slow; if slower than
16 a second, most people will fail to hear them, and the same is true if
they are more rapid than about 40,000 a second. Between these limits
vibrations that strike upon the ear are heard as sounds.
Differences in vibration rate between one sound and another can be
recognized by the ear; the difference is a matter of pitch. By the pitch
of a tone we mean the vibration rate which it has. More rapid vibrations
give tones of higher pitch; a slow rate means a low pitch. Middle C on
the piano has a rate of either 256 or 261 a second according to the
system used by the tuner. The human voice has an extreme range starting
with the lowest note that the bass voice can compass with a rate of
about 80 vibrations a second, to the highest note that famous sopranos
can attain at about 1,400 a second. There is a record of a singer who
could achieve a tone with a rate of 2,100 a second, but this has not
been duplicated so far as is known. Of course no single voice can cover
more than a fraction of this range. Most men produce all their tones at
rates of between 90 and 500 a second, and women between 200 and 800 a
second. Not every different vibration rate is heard as a tone of
different pitch; the ear is not sensitive enough for that. The interval
between one note and the next includes several vibrations, more the
higher one goes in the scale. A perfectly true tone has exactly the rate
called for; a departure of one or two vibrations a second may not be
noticed, but if the error is greater the singer is sharping or flatting
his tone, according as he is above or below the true rate. A note that
has just double the rate of another one is said to be its octave. For
convenience the interval of the octave has been split up into twelve
tones, and all our music is constructed on that basis.
It is evident that the ear must be a very complicated organ; not only
must it perceive differences in pitch, as just indicated, but
differences in loudness must also register differently. More than that,
the ear has to be able to deal with sounds made up of a great many tones
coming into it all at once. When we listen to an orchestra or band, the
waves that strike our ears represent the commotion set up in the air by
all the instruments together. It is a remarkable fact that in this case,
instead of getting a meaningless jumble, we actually get a blend of
tones from which, if we are sufficiently musical, we can pick out the
individual elements.
The ear consists, in the first place, of a vibrator that will respond
accurately to any vibration rate or combination of vibration rates
within its range, and secondly of a sensitive apparatus that is acted
upon by the vibrator. The vibrator must respond freely to feeble
impulses, and, what is of prime importance, to any vibration rate as
readily as to any other. Almost all elastic bodies have a preferred
vibration rate; that is, they will respond better to some rates than to
others. About the only exception to this rule
[Illustration: DIAGRAM OF THE EAR
_A_, auditory canal, leading to the eardrum _B_; _C_, cavity of the
middle ear, communicating by the Eustachian tube with the throat _D_ and
containing the ear bones; _E_, semicircular canals; _F_, true hearing
organ; _G_, auditory nerve. (“The Human Mechanism,” by Hough and
Sedgwick.)]
is in the case of membranes that are not tightly stretched. A stretched
membrane, like a drumhead, has its own vibration rate, but one that is
not on the stretch is able to vibrate at almost any rate. This fact is
taken advantage of in the telephone and the phonograph, both of which
depend on being able to vibrate at various rates almost equally well. In
the ear, also, an unstretched membrane is the vibrator. We are familiar
with it as the eardrum. It is located at the bottom of the ear canal,
but cannot be seen by looking therein, because of a slight curve at
about the middle. When the ear specialist wants to examine the eardrum
he thrusts a small metal tube into the canal. This straightens it out
enough to bring the drum into view.
The eardrum does not act directly upon the sensitive hearing apparatus,
but its vibrations are transmitted across a space known as the middle
ear. The necessity for this space is found in the fact that atmospheric
pressure is not constant, but changes frequently from day to day,
besides falling off as one ascends higher above sea level. The free
action of the eardrum depends on its not being stretched; if there were
no means of readjustment it might be properly set for one air pressure,
but greater pressures would bulge it inward, putting it on the stretch,
and so cutting down its ability to respond to a wide range of tones. The
middle ear, which is the space behind the drum, connects with the
outside air by a tube leading from it to the back of the throat, which
latter communicates freely with the air through the nose, as well as
through the mouth whenever it is open. The tube is known as the
Eustachian tube. Its walls are ordinarily collapsed, so it is not an
open passage, but every time one swallows the tube is pulled open, thus
allowing differences in air pressure on the two sides of the eardrum to
equalize. Whenever one ascends a high hill quickly, as by train or
automobile, or even in going to the top of a high building by elevator,
the difference in air pressure behind and in front of the eardrum can be
felt. The sensation is disagreeable, and there is definite impairment of
hearing. Repeated swallowing gives relief.
The vibrations of the eardrum are transmitted across the space of the
middle ear by a chain of three tiny bones; these are very irregular in
shape, and are attached to one another in such a way that every
movement of the drum is followed exactly as to time and direction, but
with reduced size and increased power. The hearing apparatus, which is
part of the internal ear, but not the same part as makes up the
equilibrium organ, contains liquid which is set moving by the last of
the chain of bones, and this liquid acts upon the actual sensitive cells
which make up the sense organ proper. These latter are arranged in an
exceedingly complicated fashion. There are various theories as to the
precise manner in which the vibrations of the liquid in the inner ear
arouse the sensitive cells. It is thought that different cells respond
to tones of different pitch, but exactly how this is accomplished is not
known.
Deafness may result from failure of the sensitive inner ear to respond,
or from poor transmission of vibrations across the middle ear by the
chain of bones, or by interference with the freedom of action of the
eardrum, or by preventing the air waves from striking upon the drum.
Injury to the inner ear is rare, because of its secure position within
the bone. A common form of deafness is the result of hardening of the
connections between the ear bones, so that the chain no longer follows
well the vibrations of the drum. This usually begins to come on during
the twenties or thirties, and causes almost complete deafness by the age
of fifty. In most if not all cases it is hereditary. Interference with
the action of the eardrum may be due to the partial destruction of the
drum itself. Scarlet fever and measles are particularly likely to leave
the drum in a delicate condition, and any strain upon it then may
rupture it beyond repair. Continuous closing of the Eustachian tube, by
preventing equalization of air pressure on the two sides of the drum,
causes partial deafness. Inflammation accompanying a cold may cause
this, or the growth of adenoids in the back of the throat. Adenoids will
be described later; here they are mentioned only because they may press
upon and close the Eustachian tubes. There is danger from the closure of
the tubes by inflammation, because the inflammation may creep up to the
middle ear and cause serious trouble, both there and in the mastoid
region adjoining. Earache in children should be carefully watched, since
it usually means that the middle ear has been invaded by the same
inflammatory condition that is present in the throat in colds, and may
do serious damage to the delicate structures there. Often in children,
and sometimes in adults, the hearing is impaired by the accumulation of
wax in the ear canal. This wax is a sticky secretion that serves to
catch particles entering the ear canal and to prevent them from striking
against the drum. Unless the ear canals are washed out frequently with
_hot_ water the wax accumulates and hardens into a plug which closes the
ear canal, shutting off faint sounds. The wax dissolves in hot water,
but not in cold, so its accumulation is to be prevented by taking pains
that hot water actually gets to the bottom of the ear canal once in a
while. Digging the wax out with hard instruments should be done only by
an expert with greatest caution, lest the drum be injured.
The last of the senses to be described is sight; this is the one of
which we make the most use ordinarily, and curiously enough is the only
one that we can turn on and off. Loud noises or penetrating smells must
be endured, but by shutting our eyes tightly we can escape sight
whenever we want to. Altogether there are three kinds of information
which the sense of sight brings to us; the first is the knowledge
simply of light and darkness; the second is the knowledge of the shape
and size of objects; the third is the knowledge of color. Nearly all
animals seem to have some power of distinguishing between light and
shade. Even the one-celled kinds, that have no eyes or anything
corresponding to eyes, behave in a way that proves them to have this
power. A lot of them can be put in a dish of water that is well lighted
on one side and in shade on the other and in a few minutes all of them
will be found to have traveled to one side or the other according as
they happen to be a light-seeking kind or a dark-seeking species. As we
go higher up the animal scale we find that parts of the body show this
same power of distinguishing light from darkness. In the case of the
common angleworm, or earthworm as it is more properly called, the front
end has the power but the rear end has not. In very highly organized
animals only the special organs known as eyes possess the power; all the
rest of the body has lost it. The next feature of sight, the ability to
perceive the shape and size of objects, requires a special apparatus,
the eye, so animals that lack eyes cannot perceive objects, although
they may be able to tell light from darkness.
In order to see an object it is necessary that a pattern or image of it
be thrown on a sensitive surface; this surface registers the details of
the pattern, and so the object is seen. What we have to do here is to
find out how these patterns or images are formed in our eyes. In the
first place we must realize that every visible object has rays of light
going off from every part of its surface in every direction. So-called
_self-luminous_ objects, like lamps or the sun, produce the light
within themselves; all others merely reflect light that falls on them
from some source. Whenever light falls on any object, unless it is a
perfect mirror, part is absorbed by the object and the rest is
reflected; that which is reflected presently strikes against another
object and is again in part absorbed and in part reflected; as this
process is repeated over and over again the light becomes so broken up
that rays of it are traveling in every direction from every point on
every object. Any spot so protected that no rays can strike it evidently
cannot reflect any out again, and such a spot will be absolutely dark.
For the formation of an image of any object all that is necessary is
that some of the rays of light from every point on the object be caused
to fall in exactly corresponding positions on the image. The simplest
possible means of doing this takes advantage of the fact that rays of
light travel in straight lines. If we inclose an incandescent bulb in a
tight box with a round hole in one side of it, every spot on the
incandescent filament will be giving off light in every direction, but
all the light will be cut off by the box except that which happens to
have the direction which takes it out through the hole. From every
incandescent spot, then, there will be a beam of light in the form of a
cone escaping from the box through the hole. The tip of the cone will be
the incandescent spot; the slope of its sides will depend upon the size
of the hole. If a screen is placed in front of the hole, all these cones
of light will strike on it, and it will be illuminated in a pattern
which is made up of all the cones from all the incandescent spots which
make up the filament, but these will overlap so much that one cannot be
told from another. Now if the hole in
[Illustration: DIAGRAM OF A PINHOLE CAMERA
Showing how clear images can be formed by the use of a hole so small
that only pencils of light can pass through it to strike on the screen
at the back.]
the box is made small enough, a pinhole, in fact, and if the screen is
placed close to the hole, the cones of light from the different
incandescent spots become so narrow that when they strike the screen
they overlap scarcely at all, and what we get is a tiny spot of light on
the screen corresponding to every incandescent spot on the filament and
straight in line with it through the hole. Here we have exactly what we
have been talking about, namely a pattern or image of an object. The
image will be upside down, because those rays from the top of the bulb
that strike the tiny hole will be below on the outside, and those from
the bottom will be above. The same thing can be worked exactly in
reverse; we can place a box with a pinhole in it in front of any object
and get an image inside the box on the back; by placing a photographic
plate or film there an excellent picture can be taken. There is just one
reason why this scheme is not used in all cameras; that is that unless
the object is very brightly illuminated indeed the amount of light that
passes through the pinhole is not enough to affect the plate or film
except on long exposure. Perfect pictures can be taken with a pinhole
camera wherever long exposures are possible, or wherever the object
shines brightly enough. This difficulty is gotten around in the ordinary
camera by gathering up all the light in a wide cone from each spot on
the object and condensing it again on the plate or film. The image is
formed just as before, but now each spot on the image includes, not only
the beam of light that comes in a straight line from the corresponding
spot on the object, but in addition that in a wide cone surrounding the
straight beam. It is naturally brighter the wider the cone; which
explains why in poor light we open the diaphragm of the camera wider
than when the light is good; a brightly illuminated object will pass
enough light through a narrow opening, but a dimmer object must have as
wide an opening as possible in order that enough may get through.
The method of condensing the light in a spreading cone so that it shall
come back to a point again is by means of a lens; not only is this true
of cameras, but also of the eye; in fact everything that has been said
thus far about cameras applies perfectly to the eye. There is one thing
about the way in which light is brought to a point by a lens that makes
the formation of images by this method troublesome in comparison with
their formation in pinhole cameras. That is that the cone of light which
strikes the lens is condensed as an opposite cone on the other side, and
since the formation of an image requires that every point of the object
shall be reproduced as a _point_ in the image, there is only one place
where the image can come, which is where the tips of the cones of light
are. This place is spoken of as the _focus_. Unless the screen or film
is exactly there the image will be made up of overlapping circles of
light instead of points, and so will be blurred. In a pinhole camera the
cones are so small that they cannot overlap, so there is no one place
where the image is better than elsewhere; in other words, there is no
necessity of focusing. The chief reason that focusing gives trouble is
that the farther away from the lens the object is the closer to the lens
will be its image; hence if the field of view consists of several
objects at different distances they will not all focus at the same
level; the distant objects will focus near the lens; the near objects
farther back. In practice this trouble is met by using a thick lens,
which has a very short focus to begin with, so that a considerable range
of distances will be covered without serious blurring, and for finer
work by adjusting the distance between the lens and the back of the
camera, where the film or plate is, so that there shall be a good focus
of the particular object that is desired.
In the eye the clear part that projects between the lids, and is called
the _cornea_, is the important lens. Just behind is the arrangement that
corresponds to the diaphragm of the camera; this is the colored part
with a round hole in the center; it is called the _iris_ and the round
hole is the _pupil_. Behind the iris, and resting right against it, is
the secondary lens of the eye, known as the _crystalline lens_. The eye
as a whole is a globe just under an inch in diameter; at the back of it,
straight behind the lenses and pupil, is the sensitive surface upon
which images are formed. This is the _retina_; it extends pretty well
around to the sides, but the part we use most in seeing is the small
portion straight in line with the pupil. The cornea by itself is a lens
whose focus is
[Illustration: VERTICAL SECTION OF THE RIGHT EYE AND ITS LIDS
_c_, cornea; _l_, crystalline lens, its margin shielded by _i_, iris;
_p_, pupil; _r_, retina; _m_, muscles that move the eyeball; _o. n._,
optic nerve. (From “Human Physiology,” Stiles.)]
longer than the length of the eyeball, and the crystalline lens by
itself also has a long focus, but the two in combination give a focus
that just corresponds with the length of the eyeball, so that the images
of all objects at a distance of eighteen feet or more fall sharply on
the retina. Since near objects focus farther away from the lens than far
objects, the effect of this is to make objects nearer than eighteen feet
out of focus. For them a longer eyeball would be needed, and it would
have to become longer and longer the nearer the object was brought to
the eye. We all know that when we look at near objects we make an
adjustment in the eyes. This is known as accommodation; for a long time
it was supposed that _accommodation_ was actually secured by lengthening
or shortening the eyeball to bring the focus right, but we now know
that the eyeball does not change in shape when we accommodate. The same
result is secured by another means, namely, by letting the crystalline
lens bulge out and become thicker. It was stated incidentally a few
pages back that thick lenses have shorter focuses than thin. So when the
crystalline lens thickens it shortens the focus of the eye, and throws
the image forward. This will locate the image of near objects on the
retina, instead of behind it, as in the unaccommodated eye. The
crystalline lens is not stiff like glass, but rather like a thick jelly;
it is in a transparent bag, or capsule, which is fastened to the inside
of the eyeball all around, and the pull on the capsule stretches it out
pretty flat, making the lens thin. There are tiny muscles inside the
eyeball, known as the _ciliary_ muscles. These are so fastened that when
they contract they pull the eyeball forward and inward, loosening the
tension on the capsule of the lens. This then bulges out, taking up all
the slack. When we learn, as babies, to accommodate for near objects we
find out just how much the capsule must be loosened to give the proper
adjustment for any distance. As people get along in years the
crystalline lens often loses its elasticity so that it does not bulge
when the capsule is loosened. After this happens vision for near objects
is no longer clear. Relief is obtained by wearing reading glasses. These
are merely glass lenses worn in front of the eyes selected so that their
focus fits in with that of the lenses of the eye itself to bring the
image of objects at reading distance sharply on the retina. It is
usually necessary to replace these glasses from time to time as the
crystalline lens becomes stiffer and stiffer and ordinary accommodation
fails more and more.
There is a disease known as _cataract_ in which the crystalline lenses
become cloudy and finally completely opaque. Of course this means
blindness, since the passage of light to the retina is interfered with.
Relief is obtained by the simple expedient of removing the opaque lenses
bodily. This is possible merely because the crystalline lens is not the
chief lens of the eye. To be sure the cornea by itself will not focus on
the retina, but a glass lens can be placed in front of it which will add
itself to the cornea and the combined lenses will. There is no
possibility of accommodation in a case like this, so the patient has to
be furnished with bifocal lenses; the main part gives clear distance
vision; the lower section gives clear vision at the reading distance.
The patient has to get along with blurred vision in the regions between.
Not all eyes are exactly the right size so that distant objects shall
focus sharply on the retina. In fact a large proportion of them are
either too long or too short. It is clear that in an eyeball that is too
long the image of distant objects will fall in front of the retina, but
near objects that happen to be at just the right distance will focus
exactly on the retina. The distance at which this happens depends, of
course, on how much too long the eyeball is. Persons that have unduly
long eyeballs are, therefore, nearsighted. The condition is called
_myopia_. The correction for it consists in the use of lenses in front
of the eyes that instead of shortening the focus shall lengthen it.
Concave lenses, namely, those that are thick at the edge and thin in the
middle, will do this, and these are the kind that are worn by
near-sighted people.
When the eyeball is too short the image of distant objects falls behind
the retina, and of course that of near objects tends to fall farther
back yet. Since by
[Illustration: Photo, Fifth Avenue Hospital
SOFT, RESTFUL COLORS AND ATTRACTIVE SURROUNDINGS ARE CONDUCIVE TO GOOD
SPIRITS AND THEREFORE TO HEALTH]
[Illustration: Copyright, Paul Thompson
THE MAGNET IS HERE PUT TO THE UNUSUAL SERVICE OF REMOVING IRON FILINGS
FROM THE EYE]
accommodation the focus can be thrown forward, most persons with short
eyeballs can see distant objects clearly by accommodating for them, and
near objects that are not too near by extreme accommodation. For this
reason _hyperopia_, as this condition is called, is usually not
discovered until the person begins to feel the strain of the constant
accommodation that is necessary whenever the eyes are open. Eyestrain
usually shows itself in headaches; in fact, so large a proportion of
headaches come from this cause that anyone who suffers from them at all
frequently should have his eyes examined by a competent oculist. Relief
for hyperopia is by means of glasses that shorten the focus and thus
bring the image of distant objects forward to where the retina is in the
short eyeball.
[Illustration: EYE LENS--NORMAL AND DEFECTIVE
_A_, normal eye; _B_, myopic eye; _C_, hyperopic eye (From Martin’s
“Human Body”)]
There is one other defect of vision that is so common as to call for a
word; this is _astigmatism_. It is the condition in which the cornea is
not curved equally in all directions; the vertical curvature may be
greater or less than the horizontal. The effect is that points on
objects do not focus sharply as points in the image, but as little
elliptical spots. If one adjusts the accommodation so that the top and
bottom edges of objects are sharp the sides will be blurred and vice
versa. Usually the blurring is not great enough to be noticeable, but
only enough to make the person unconsciously dissatisfied with the
accommodation. He, therefore, constantly tries to improve it by changing
the tension of his ciliary muscles, and so brings on eyestrain. The
correction for this condition is glasses that are not equally curved in
all directions, but so selected that their less curvatures shall fit in
with the greater curvatures of the cornea. In fact, glasses that are
curved only in one direction are usually used, this curvature being just
enough to bring up the total curvature in that direction to equal the
other curvatures of the cornea. It ought not be necessary to give
warning that only competent persons should be allowed to examine and
prescribe for the eyes. Great skill is needed to determine accurately
just how far from correct the eyeball is, and unless this is known there
is no means except guesswork by which to decide on a prescription.
The third feature of vision is the perception of color. Color is to
light what pitch is to sound; that is, it depends on the vibration rate
of the light waves. Light, as already explained, is one of the forms in
which energy reaches the earth from the sun. Heat is another form. Both
are portions of a great energy stream to which we give the name of
_radiant energy_. This, as it comes from the sun, is made up of a
mixture of vibrations having almost every imaginable rate, except that
the slowest are many times faster than the highest pitched sound. At a
certain rate, and one that for these vibrations ranks as slow, the
energy is what we know as heat, and over a considerable range it
continues to be called heat; more rapid vibrations, and they are so
rapid that they have to be expressed in trillions per second, cause the
effect on our retinas that we call light. The slowest that we can see
give the sensation of red; the most rapid the sensation of violet; the
other colors of the rainbow, which in order after red are orange,
yellow, green and blue, are vibration rates between those that give red
and those that give violet. It will be noticed that only six colors are
given for the rainbow instead of seven. This is because there is not
enough real difference between blue and indigo to justify making them
separate colors. The distinction was made at the time when it was
supposed that there was something specially wonderful about the number
seven, which made it necessary that every important feature in nature
should show that number. We now know of no reason why seven should have
virtue over any other number. When all the vibration rates are mixed
together, as they are in the sunlight, the sensation is white. There are
other mixtures of colors that give white also, but they are not exactly
equivalent to the white of sunlight, as is proven when one tries to
match colors under artificial light. A great deal of labor has been
devoted to the attempt to get an artificial light that shall be
practically equivalent to sunlight, and only lately have good results
been obtained. When no light enters the eye the sensation is of black,
and it is worth while to note that so far as our sensations are
concerned black is as much a color as white or any other,
notwithstanding the fact that no light falls on the retina when the
black sensation is being felt. Contrary to ordinary belief, blind
persons who are blind because their eyes have been destroyed do not see
black all the time; they simply have no sensations at all from the eyes.
On the other hand, persons blind because of cataract do see black,
because in them the retinas are still present, but no light falls on
them.
The perception of color is very complicated, and not at all well
understood. Persons who do not have the same color perception as most of
us are called color-blind, and by learning some things about
color-blindness we shall best get an idea of color perception itself.
About four men in one hundred have defective color sense; the proportion
in women is only about a tenth as great. By far the commonest type is
one in which neither red nor green is seen correctly, but both are seen
as neutral tints, and in many cases look so much alike that the person
cannot tell one from the other. The practical importance of knowing
whether or not this defect is present is seen when we think that red and
green lights are used more than any other colors in signaling, so that
railroad men and others who work by signals must have normal color
sense. It has been discovered that even people that have normal color
vision are color-blind in the margins of the retina. This can easily be
demonstrated by bringing a red or green disk slowly around in front of
the eye of a person who, meanwhile, keeps looking straight ahead. He
will see the disk out of the corner of his eye some time before he can
tell what color it is. In fact, if a red or green disk is used, he
usually will not be certain as to the color until it is almost straight
in front. Blue or yellow disks can be told with certainty much farther
out, but even these colors are not perceived clear to the edge of the
field of vision. These experiments show that the retina becomes more and
more highly developed as an organ for perceiving color as we get closer
and closer to the center; at the extreme edges there is no color sense
at all, but only the primitive ability to tell light from darkness;
closer in blue and yellow are distinguished and not red and green; only
in the central part are all colors clearly seen.
In addition to the kinds of information which have been described thus
far, that the distance senses bring in, there is another kind, fully as
important as any in our actual use of our senses; that is information as
to the “direction from us” of the object or objects which are arousing
the sense. We can get this through all the distance senses, but much
more perfectly in the case of sight than in the others. We locate the
direction of objects that we smell by turning the head this way and
that, sniffing meanwhile, and noting the position in which the odor is
caught most clearly. Animals with a keen sense of smell, like dogs, can
locate directions very accurately by this means. In the case of hearing
the method is to turn the head until the sound is equally loud in both
ears. We would expect that a person who was hard of hearing in one ear
would never be able to locate sounds by this method; but, as a matter of
fact, such persons unconsciously allow for the difference in hearing in
the two ears, and so can judge the direction of sounds about as well as
any of us. Animals, like horses or rabbits, that have very movable outer
ears, undoubtedly can locate sounds much more accurately than we can.
Our outer ears are of almost no use in hearing; persons who have had the
misfortune to lose them hear practically as well as anyone.
[Illustration: DIAGRAM SHOWING HOW DISTANCE AFFECTS THE SIZE OF THE
IMAGE]
We locate directions with the sense of sight with perfect accuracy,
because unless the image of the object we are looking at falls on the
center of the retina it is not seen clearly. The only way to make the
image fall just there is to look directly at the object. The muscle
sense in the eye muscles is extremely delicate, so that if the eyes are
rolled at all in looking at anything we know it and can judge, also, how
much they are turned from the straight position. In this way we are able
to tell exactly the direction from us of any object we can see.
We can judge the _distance_ of a near object very accurately by noting
the degree to which the two eyes have to be turned in in order to see it
clearly with both. We are quite unconscious of this means of making the
judgment; all we know is that we can tell. It is easy to prove that it
depends on the two eyes by closing one and trying to make movements that
depend on accurate knowledge of distance. A good example is threading a
needle sideways. With both eyes open this can be done fairly easily, but
with one shut it cannot be done at all, except by chance. Objects so far
away that the eyes are not turned in perceptibly in looking at them are
judged as to distance wholly on the basis of their size. It is clear
that the actual size of any image on the retina will depend in part on
how large the object is, and in part on how far away it is. If an object
that we know to be large casts a small image on the retina, we conclude
that it is far away. It follows that unless there are some familiar
objects in view, judgments of distance are not at all trustworthy. A
good illustration is in looking up a bare hillside, and trying to
estimate the distance to the top. If, while this estimation is being
made, a man or horse suddenly comes into view at the top, the man or
horse will nearly always appear unexpectedly large, showing that the top
of the hill is not actually as far away as it was judged to be.
The possession of two eyes instead of one is an advantage to us in
another way, in addition to helping in the estimation of near distances.
This is in making objects appear solid, or in other words, in helping
the estimation of depth. When we look about us we have no difficulty in
realizing that some objects are near and others far, and that the
objects themselves have some parts that are nearer to us than other
parts. A great many things assist us in this realization. First and
foremost comes that which is known in art as perspective, namely the
tendency of distant objects or distant parts of objects to appear
smaller than those that are near. This can best be illustrated in the
case of parallel lines extending away from the eye, as when one stands
on a straight railroad track and looks along it. Although we know
perfectly well that the rails are the same distance apart all along, if
we were to believe our eyesight implicitly we should think that they
came gradually together. It is on account of this matter of perspective
that drawings of solid objects must show supposedly parallel sides
nearer together at the far end than at the near. Besides perspective
there are the shadows to be taken into account. Only solid objects cast
shadows, so if we see a shadow apparently cast by any object we
naturally conclude that it is solid. Both perspective and shadows can be
and are used by artists in making drawings and paintings look real. In
fact, they have almost no other means of doing this. As we all know,
even the cleverest paintings do not give an impression of depth equal to
that which comes from actually looking at solid objects. This is because
of help we get from the two eyes in the latter case. The reason for the
difference is that when we look at a solid object with both eyes the
view we get with one eye is not exactly the same as with the other; we
see a little farther around on the left side with the left eye, and on
the right side with the right eye. The combined view with the two eyes
gives us an impression of solidity that cannot possibly be had when the
view with the two eyes is exactly the same, as when we look at a
picture. The only way in which the impression of depth can really be
gotten in a picture is by using the familiar method of the stereoscope,
where two pictures are taken simultaneously by two cameras, placed a
little farther apart than the two eyes; and then the two are looked at
together through a special pair of prisms.
By means of the three distance senses, smell, hearing, and sight, we are
informed pretty completely as to what is around us. All three give an
idea as to the direction from us of objects; although sight does this
better than either of the others. Sight, also, lets us know accurately
as to the distance away of objects, provided they are fairly near. Smell
and hearing, as well as sight, may give us some idea as to the distance
of far objects, but only when we are dealing with familiar sensations. A
very faint smell, or faint sound, means a distant object provided we
know enough about the source to know that if the object were near the
sensations would be keener. Our judgment of distant objects by sight is
better than this, but not by any means perfect. When we contrast the
distance senses with the contact senses we see at once that the great
advantage coming from the possession of distance senses is in the time
that is permitted for action. In the chapter on contact senses we
emphasized the fact that the response to them must be immediate; there
is no time to pick and choose. When we can learn of the presence of
objects before they reach us, and something of their direction and
distance as well, we can usually take time to select the most fitting
course of action. We do not have to jump into a mud puddle to escape an
automobile if we see it soon enough. We shall learn in a later chapter
how this opportunity of choice is wrapped up with our development into
highly intelligent animals.
CHAPTER X
THE NERVOUS SYSTEM AND SIMPLE NERVOUS ACTIONS
In the second chapter we saw that to make our muscles act in accordance
with the information brought in by the sense organs some means of
communication between them is necessary; we saw, also, that this means
consists of the nervous system. Now that we have learned something about
both muscles and sense organs we are ready to look into the way in which
communication between them is carried on. First of all, we must realize
that living protoplasm does this. The nerve cells are alive and have
their basic metabolism just as do all other living cells. They also have
their functional metabolism; but in them, instead of taking the form of
forcible motion, as in muscles, or of the manufacture of special
materials, as in gland cells, it takes the form of the transmission of a
disturbance from one part of the cell to another. An interesting and
important fact about this transmission of disturbances is that the
actual amount of functional metabolism required by it is very small.
Only by the most careful measurements has it been shown that nerve cells
that are functioning have a greater metabolism than those that are at
rest. For a long time it was thought that a nerve cell acted very much
like a telephone or a telegraph wire, transmitting some kind of a
disturbance which was set up in it, but not having any active part
itself in the process. We now know that the special activity of nerve
cells is a form of functional metabolism, just as is the special
activity of muscle cells or gland cells.
The nerve cells have to make communication between sense organs and
muscles, and, as we have already seen, these are often quite a distance
apart. It is necessary, therefore, that the nerve cells be long enough
to reach over these distances. As a matter of fact, it is not necessary
for single cells to have this great length, because it is possible for
them to be arranged end to end, making a path of living protoplasm
consisting not of one cell but of a chain of them. If we look at a nerve
cell under the microscope we see that it is made up of a little central
mass of protoplasm to which has been given the name of “cell body.” From
this cell body extends a tiny thread of living protoplasm. This thread
is called the _axon_. It is so very slender that it cannot be seen
except under a powerful microscope, and yet in our own bodies and the
bodies of all large animals many of these are three feet or more in
length without a break. This tiny protoplasmic thread, the axon, was
formed originally by growing out from the cell body. As it grew it
became surrounded by a sheath, which probably gives it strength and
decreases the danger of its being broken. Another thing which helps to
keep the axons from being injured is that they are always in bundles.
Instead of one of these very slender axons lying all by itself, it will
be bound up with several hundred others; the arrangement is similar to
that in a telephone cable, where a great many single wires are bound
together in the large and very strong cable. The living protoplasm of
the nerve cell has a gray color, so that wherever this shows we have
what is commonly called gray matter. We saw a moment ago that every axon
is inclosed in a sheath. Some of these sheaths are transparent, so that
the gray color can be seen underneath, but most of them have a layer of
white material, which makes them look white instead of gray. The bundles
of axons corresponding to the telephone cables make up what we call the
nerves. Nearly all nerves are white in color because of the white
material in the sheaths.
The sense organs, as we have seen in Chapters VIII and IX, are some of
them inside the body, others spread over the surface of the skin, and
the rest in the special sense organs, like the eyes or the ears. A very
complex organ, like the eye or the ear, has thousands of axons leading
from it. In the case of the eye these are grouped into a large nerve
leading away from it at the back, which is called the optic nerve. There
is a similar large nerve leading from the ear. When any sense organ is
acted upon, as when light falls in the eye or sound on the ear, it
starts a disturbance in some or all of the axons leading away from it.
As we have said over and over, the purpose of the nervous system is to
arouse the muscles to activity, and to guide them in that activity. We
must ask next, then, how the nerves are distributed to the muscles. If
we dissect the body of any animal or bird, we can find nerves passing to
various parts. For example, a large nerve goes down each leg; this nerve
subdivides here and there. Since we know that the nerve consists of a
great many axons bundled together, we will realize that this
subdivision is not a real branching, but simply a passing of some of
the axons away from the main trunk along the smaller stem. Some of these
smaller stems can be traced to endings in the skin; these contain the
axons connecting with sense organs. Others lead directly into muscles.
Some of these axons may also connect with sense organs, since, as we
have already seen, every muscle has embedded in it the organs of muscle
sense, but in addition any nerve that leads to a muscle contains a great
many axons which pass directly to the muscle fibers. These are the axons
by which the muscles are aroused to activity. It is a general rule of
the nervous system that no nerve cell extends without a break from any
sense organ to any muscle fiber. The axon which communicates with the
sense organ belongs to one nerve cell; the axon which connects with the
muscle fiber belongs to a different nerve cell. The first is called a
_sensory_ nerve cell, the second a _motor_ nerve cell. Some idea of the
appearance of these cells can be gotten from the figures on page 126.
It will be seen that the cell body of the sensory cell appears to be off
on a little side branch. As a matter of fact, the branch is double, so
that when a nervous disturbance passing along from the sense organ comes
to the beginning of this branch it can pass up to the cell body and then
out from the cell body along the second part of the branch, and so along
the other part of the axon. This part of the axon is seen in the figure
to have several branches; these are really branches of protoplasm and
not separate axons coming off, as in the case of the nerve trunk. The
use of these branches we shall see in a moment. Also at the tips of
each branch there is a tiny feathering. We shall explain this
presently. Let us look first at the figure of the motor nerve cell. This
has a cell body and long axon, and, besides these, has a great many
short protoplasmic branches sticking out in all directions from the cell
body. Since a nervous disturbance to get from a sense organ to a muscle
has to pass over a sensory nerve cell, and also over a motor nerve cell,
evidently there will have to be some point at which it leaves the
sensory cell and gets into the motor. This is accomplished by having the
tiny feathering at the tip of the sensory cell interwoven with the fine
processes projecting from the body of the motor cell. This arrangement
we may call a nerve junction. In the whole body there are, of course,
millions of these nerve junctions.
[Illustration: MOTOR NERVE CELL
SENSORY NERVE CELL
(From Martin’s “Human Body”)]
We have just described the simplest arrangement of a nerve path from a
sense organ to a muscle; it consists of the sensory nerve cell, a nerve
junction, and a motor nerve cell. This arrangement will answer where the
sense organ and the muscle are in the same part of the body, but it may
happen that the sense organ is in one part of the body and the muscle is
in a distant part, as, for example, the eye and the muscles of the hand.
To make these distant connections there must be additional nerve cells,
and these we find in the body in the form of a kind of nerve cell that
serves as a connecting link between sensory cells and motor cells. It
may be called simply a connecting nerve cell. In appearance it is like
the motor nerve cell, except that it has many branches which do not
terminate in muscle fibers, but in fine feathering like that at the tips
of the sensory nerve cell. When these connecting cells are present in
the chain, the arrangement is as follows: from the sense organ the
sensory nerve cell will pass just as previously described, the
feathering at the tip will form a nerve junction with a connecting cell
instead of with a motor cell. This connecting cell also has feathering
at the tips of its branches, and these featherings will form a nerve
junction with another nerve cell. This may be a motor cell, in which
case the pathway is completed, or it may be another connecting cell,
which in turn may lead into motor cells or connecting cells.
The nervous system is made up, then, of chains of nerve cells. Now why
is this arrangement present? In very many cases sense organs and muscles
are side by side or within a short distance of each other; why does not
a single nerve cell reach directly from the sense organ to the muscle?
The
[Illustration: CONNECTING NERVE CELL
(From Martin’s “Human Body”)]
answer is simple, if we think for a moment of how the body works. The
information that comes in by way of particular sense organs cannot
always be used to arouse particular muscles to activity. Things that we
see will sometimes cause us to move one hand, sometimes another hand,
sometimes the legs, sometimes muscles of the head, and so on. This means
that the eye is able to make connection with a very large number of
different muscles. The same thing is true of the other sense organs. The
body could not possibly work as it does, if certain sense organs
connected with certain muscles and no others. As a matter of fact, it is
not an exaggeration to say that the proper working of the body requires
any sense organ to be able to make connection with any muscle. It might
be possible to do this by giving every sense organ as many sensory nerve
cells as there are muscles, but this would be as bunglesome as to
attempt to provide every business house in a large city with a separate
telephone wire to every other business house.
The arrangement of the nervous system is very much like that of a city
telephone system. The sensory nerve cells all lead into a part of the
nervous system to which is given the name of the _Central Nervous
System_, just as the telephone wires all lead into a “central exchange.”
From this central nervous system or “exchange” all the motor nerve cells
extend to the muscles. There is one important difference between the
arrangement of the nervous system and that of the telephone exchange;
namely, that the nervous system is a “one-way” system. As we all know, a
telephone instrument can be used either for sending or receiving and the
same wires conduct the messages in both directions. This is not true of
the nervous system. The messages from the sense organs pass into the
center by way of the sensory nerves and out from the center by way of
the motor nerves. The central nervous system operates as an “exchange,”
connections can be made from any sensory cell to any of the motor cells.
Since this ability of the central nervous system to make connections
here and there within itself is about the most important of all our
nervous activities, as we shall see shortly, we must try to form an idea
of how it is done. If we look again at the figures of the sensory and
connecting nerve cells, we shall note that both kinds are branched; the
tip of every branch makes a nerve junction. This means that every
sensory cell, for example, has as many outlets as it has branches. If
every one of these outlets were to communicate directly with a motor
nerve cell, the number of connections that
[Illustration: DIAGRAM OF NERVE CELL CONNECTIONS
_S C_, spinal cord; _S_, sense organ; _M_, muscle; _a_, sensory nerve
cell; _b_, connecting nerve cell; _c_, motor nerve cell]
this sensory cell could make would depend on the number of its branches;
but, as a matter of fact, most of the branches from the sensory cells
make nerve junctions with connecting cells, and not with motor cells
directly. These connecting cells in turn are branched and many of these
branches lead again to connecting cells, so we see that the number of
connections that a single sensory cell can make quickly becomes very
large. The arrangement is shown in the accompanying diagram. Not only
does one sensory cell have in this way the possibility of nerve
connection with all the muscles, but the reverse is also true; namely,
that every muscle has the possibility of being connected with every
sense organ. This means that there must be a number of nerve junctions
connecting with the cell body of each of the motor nerve cells. If we
look back at the diagram of the motor nerve cell, we shall see that it
has a great many tiny branches leading off it. These are numerous enough
to enable the feathery tips of a great many sensory or connecting nerve
cells to interweave with them, and so enable any motor cell to be acted
upon from a great many different directions.
The central nervous system, which is the place where all these
much-branched pathways are and where all the nerve junctions are
located, is made up of two chief parts, the brain and the spinal cord.
The brain is inside the skull and the spinal cord is an extension down
the back. It lies in the tunnel made up of the arches of the bones of
the vertebral column as described in Chapter VI. The cables which
contain the axons of both sensory and motor nerve cells extend from the
brain or from the spinal cord out to the different parts of the body
where the sense organs and the muscles are located. They start as large
nerve trunks which divide and subdivide as they get farther and farther
away from the central nervous system. The large nerve trunks are
arranged in pairs--those that spring from the brain are called _cranial_
nerves; those that spring from the spinal cord are called _spinal_
nerves. The cranial nerves, with the exception of one, lead only to
points in the head or neck, and so are short; the spinal nerves on the
other hand reach to the various parts of the trunk or down the arms and
legs, so that some of these may be three feet long or more. An idea of
the arrangement of the nervous system is given in the accompanying
figure.
A good way to realize the actual working of the nervous system is to
take a particular action and follow it through. Suppose a barefooted boy
steps on a sharp thorn. The thorn arouses some of the sense organs in
the sole of the foot. These in turn start a disturbance in the sensory
nerve cells which pass up the leg to the lower end of the spinal cord in
the small of the back. Within the spinal cord the sensory cells branch
and the disturbance set up by the prick of the thorn spreads all over
these branches to their tips. Some of the nerve junctions thus affected
lead to muscles which will cause the foot to be jerked up; others
communicate with connecting nerve cells which extend all the way up the
spinal cord and into the brain and make the boy aware of the fact that
he has stepped on the thorn; still others may make connection with
muscles which would cause him to sit down and look at the bottom of his
foot; still others may lead to the vocal muscles and the tear glands,
causing the boy to cry. Of course these are not the only possible nerve
connections; every muscle in the body might theoretically be aroused to
action as the result of the stepping on the thorn. As a matter of fact,
there is a condition in which this will happen. The drug strychnine has
such an effect upon the nervous system that the stimulation of any sense
organ actually does arouse all
[Illustration: DIAGRAM ILLUSTRATING THE GENERAL ARRANGEMENT OF THE
NERVOUS SYSTEM
(From Martin’s “Human Body”)]
the muscles in the body, giving what is called a convulsion. There are
some other poisons which may act similarly. Convulsions are not at all
uncommon in young children. The way in which a convulsion is produced is
by the spreading of the disturbance all over from a single sense organ.
What we have, then, in the nervous system is an arrangement whereby
under special conditions a nervous disturbance can pass from any sense
organ to any or all of the muscles, but under ordinary conditions the
disturbance spreads to a particular muscle or group of muscles only. It
is evident that the nervous system would be of no use at all, if this
latter arrangement did not exist. In order for our muscles to serve us,
they must act in obedience to information brought in by the sense
organs, and this can happen only when certain groups of muscles work in
accordance with the information brought in by certain sense organs or
groups of sense organs. We explain the behavior of the central nervous
system by saying that there are preferred pathways through it, or, to
put it in a slightly different way, when a nervous disturbance spreads
over any nerve cell it extends equally over all parts of it, but does
not pass with equal ease over all the nerve junctions. Some of the nerve
junctions allow the disturbance to pass more readily than others, and it
is this difference in the ease of passing the nerve junctions that
determines which pathway the disturbance shall follow. We know no other
means by which this picking out of particular paths from the huge number
of possible paths could be accomplished.
What we are describing now is the simple foundation on which all our
nervous activities rest. For that reason we are not taking up at this
point the working of the brain, but only the direct connections between
sense organs and muscles. In some very low animals the whole nervous
system is made up of such simple connections.
[Illustration: DIAGRAM ILLUSTRATING A REFLEX
An object _O_ suddenly appears in front of the eye. Its image, formed on
the retina _R_, a sense organ, starts impulses along the fibers of the
sensory nerve cell _s_, which in turn stimulate the motor nerve cells
_m_. These in turn stimulate the appropriate muscles of the eyelid,
compelling a wink. (After Hough and Sedgwick, “The Human Mechanism.”)]
A nervous activity which consists of the passage of a disturbance from a
sense organ to a muscle is called a _reflex_. We have many examples in
ourselves; if we inadvertently touch something hot, the hand is jerked
away. Tickling the soles of the feet in one who is asleep will cause
them to be drawn up; irritation in the throat causes us to cough, or in
the nose to sneeze; the flashing of a bright light into the eye compels
us to wink; all these are examples of the direct passage of nervous
disturbances from sense organs to muscles. In every case the path is
from the sense organ over the sensory nerve cell to some point in the
central nervous system, then either by a direct nerve junction to a
motor nerve cell or over one or more connecting nerve cells to a motor
nerve cell and so to the appropriate muscles. These reflex actions
follow the arousing of the sense organ with no more delay than is
required for the passage of the disturbance over the nervous pathway.
There is a delay of a small fraction of a second at every nerve
junction, which makes some reflexes slower in their action than others.
For example, there is a reflex known as the knee jerk; this is an
outward kick which results from a sharp blow on the front of the leg
just below the knee. The kick follows so closely after the blow that
there cannot be more than one or at the most two nerve junctions in the
pathway. The reflex of winking, on the other hand, takes several times
longer, although the eye is much nearer the central nervous system than
is the place on the leg which is struck in arousing the knee jerk. Since
the actual length of nerve to be passed by the disturbance is much
shorter in the winking than in the knee jerk, while the time for the
reflex is a good deal longer, we con
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