The Nervous System
( Originally Published 1917 )
The Function of the Nervous System.—The human body is a delicate and complicated machine, every part of which must act in harmony with every other part, if the whole machine is to do its work properly. The heart must beat just so fast and the walls of the blood vessels must exert just so much pressure upon the blood. The lungs must be expanded so as to inhale the air just so often. The intestines must move the food along, and the glands must pour out the different digestive juices, each in the right amount. The kidneys must get rid of the wastes, and a score of other processes must proceed in constant harmony. If any part of the body works too fast or too slowly, the balance of the machine is upset and disease results; for disease is failure of the living machine to do its work properly.
This, however, is only a part of the story. It would not do for the body to work at the same rate all the time, like an ordinary lifeless machine. It has to change its behavior to meet different conditions. Suppose you have been sitting still and then get up and play a game of tag or baseball. Your muscles begin to work, and at once your whole body must adapt itself to the new activity. The heart must beat faster to send to the muscles the blood they need for their extra activity. The breathing must be deepened and quickened to supply more oxygen. The blood vessels in the skin must be made larger, and the sweat glands must begin to work so as to get rid of the waste products. In order that all these changes shall be properly made, there must be some-thing to control and direct all the parts of the body. This power which governs the body and makes its teamwork possible is the nervous system.
The Central Nervous System.—The brain, the spinal cord, and the nerves that run out from them are the chief organs which carry out this important work. They constitute what is called the central nervous system.
How the Nervous System Works.—You have perhaps seen a switchboard in a telephone exchange. From this switchboard, hundreds of wires run out to buildings in every part of the town. "493 Broad" calls up and wishes to talk to "287 Main"; and the operator makes the connection, so that the people in these two houses can speak to each other. The nervous system of the body works in somewhat the same way. The brain and the spinal cord are like the switch-board; from them nerves run to all parts of the body, like tiny telephone wires, bringing in messages as to what is happening and sending out messages to control and direct action.
We do not know just how messages are carried along the nerves, but we do know that each nerve serves a certain part of the body and carries a certain kind of message. A prick on the finger hurts because special sense organs in the skin are connected with nerves which cause sensations of pain. The nerves from the eyes cause sensations of light; if these nerves were cut or injured behind the eye, there would be no pain but a sensation like a flash of light. In the same way, the nerves which control the contraction of muscles cause definite movements in particular parts of the body. For instance, in a frog there is a certain nerve which, if pinched, makes the leg kick, even after the frog is killed and dissected.
Special parts of the brain and cord govern the actions of special parts of the body. A blow on a certain part of the head may, for instance, cause paralysis of the legs, so that the person injured in this way becomes a cripple and cannot walk, while an injury to another part of the head may take away the power of speech.
The Nerves.—The nerves are shining white threads running out from the brain or the spinal cord. These nerve trunks subdivide again and again, and branch out, like the branches of a tree, till they reach every part of the body. The largest of the main nerve trunks, which runs down the leg, is the size of a large lead pencil.
The nerve trunks are bundles of fine threads or nerve fibers, which are of two kinds: sensory fibers which bring in sensations from the various parts of the body to the brain and cord, and motor fibers which carry out the messages causing action of the muscles or other organs.
If you could trace one of the tiny nerve fibers (perhaps one-thousandth of an inch in diameter) in a main nerve trunk, you would find in most cases that it ran continuously, like an independent little telephone wire, right into a special nerve cell in the brain (or spinal cord). These nerve cells are situated in swellings of the nerve trunks called ganglia. The nerve fiber that controls the muscles of the foot can be followed to the lower part of the spinal cord in the back, for a distance of over three feet.
The Brain.—The brain is a large organ, weighing nearly three pounds, which almost fills the inside of the skull. It is shaped somewhat like an oval loaf of bread, with a deep groove along its upper side, and has a very irregular surface with many smaller grooves and wrinkles in it. In man, these grooves and wrinkles are much more fully developed than in the lower animals. It is interesting to note, also, that the size of the brain varies greatly in different animals, being usually much larger in the more intelligent species.
The brain is largely made up of the complicated net-work of white nerve fibers which run into it. Large bundles of these fibers enter the brain from the eyes and from the nose and ears, while at the bottom of the skull there is an opening for the great number of nerves which enter from the spinal cord.
The Gray Matter.—The most interesting thing in the brain is the gray matter, which makes up its outer layers. This gray matter contains the nerve cells into which the nerve fibers run. These cells exert certain chemical or physical effects on one another; and it is by these effects that the messages entering along the sensory fibers cause sensations, and lead to the sending out of messages of action along the motor fibers.
The Three Parts of the Brain.--There are three principal regions of the brain (see Fig. 64). The largest part, which lies over and covers up the others, is the forebrain or cerebrum. It is through the activity of this part of the brain that we are conscious of sensations such as we obtain by seeing, hearing, smelling, and tasting. The cerebrum is the seat also of all reasoning and thinking processes, and by the messages it sends out controls our voluntary actions.
The hind-brain is called the cerebellum, and below it lies the medulla (me dill' a). They control such actions as those involved in walking and breathing, which are called involuntary actions because they usually occur without any willing on our part.
The Spinal Cord.—The outside of the spinal cord is covered with a white sheath of nerve fibers, and the center is made up of gray matter like that on the outside of the brain. As the cord passes through the arches of the vertebral column, it gives off a pair of nerves at each vertebra, one to each side of the body. Each of the sixty-two spinal nerves, which run out in this way from the thirty-one vertebrae, subdivides again and again till the hundreds of fine thread-like branches finally reach all parts of the body.
The Sympathetic System.—Besides the brain and cord and the nerves directly connected with them, which make up the central nervous system, there is another important group of ganglia and nerve fibers which form the sympathetic system. Some of these ganglia lie in two chains, one on each side of the spinal column, and others are scattered through the various organs of the body. They are connected with one another and with the spinal cord by a complex network of nerve fibers.
These ganglia control, in particular, the automatic internal organs, such as the heart, blood vessels, and digestive organs, as well as the heat-regulating and sweat-regulating machinery of the skin. It is because they keep the internal organs working together, as if they were "in sympathy" with one another, that they were given the name "sympathetic system." It should be remembered, however, that the sympathetic ganglia do not make up a wholly distinct system. They are connected by special fibers with the spinal cord. They send sensory messages to it and get motor messages from it.
Reflex Actions.—Among our commonest activities are the reflex actions—actions that are caused directly by some sensory message carried to the central nervous system and there changed into a motor impulse without the action of the will. The particular work of the spinal cord and the lower parts of the brain is to serve as a kind of switchboard for the control of these reflex actions.
A reflex action may perhaps be best understood by an illustration. Suppose you put your finger on the hot stove. You draw it away as quick as a flash. You do not have to think about it; in fact, you would find it hard to keep your finger on the stove if you tried to do so.
In that short moment between the time your finger touched the stove and the time it was drawn away, a good deal happened. First the heat took effect in the skin. Then a message passed up the sensory fibers to the spinal cord to report that something was wrong at the tip of that finger. In the cord, the connection was made between this incoming nerve from the finger and the motor fibers going out to the muscles of the arm; and out along these nerves went the message which caused the arm to contract and draw away. Reflex actions are very common. Dust tickles your throat and you cough. A cinder gets into your eyes and the tears flow. You stumble and your hands go out to save yourself from falling. All these are reflexes. In each case, a message comes into the nervous system recording some condition outside. It passes to the brain or spinal cord along the sensory nerves; there, connection is made with another set of nerves; and out along these nerves goes an impulse that causes the necessary action.
It may seem to you that you pull your arm away from the hot stove voluntarily, because the nerve message, after making the connection in the cord, passes on to the brain and you are conscious of what you have done. But, in reality, the message to pull away your arm had gone out from the cord before your brain knew that your finger had been burned.
Actions that Become Reflex by Habit.—Many of the reflex actions of the body, such as winking when something comes near the eye, or coughing when something tickles the throat, are natural and do not have to be learned. Other actions, which at first require the direction of the brain, later become reflex by repetition. For example, when a child first learns to ride a bicycle, he must think of each movement (using his cerebrum). Even with all his care, his legs do not work together properly, and he loses his balance and runs into the trees along the road. Gradually, however, practice makes him more expert. He learns to keep his balance by swaying to the right if his bicycle leans to the left. After a time the process becomes quite automatic, so. that he leaps on his wheel and rides off without thinking of it at all. If there is something in the way, a message goes in from his eyes to his brain, down the cord, and out to the muscles, and he turns aside and passes the obstacle smoothly and unconsciously.
What has happened in such a case as this? A new set of reflexes has been created, which the body did not have be-fore. A new set of connections has been formed between various sensory and motor nerve fibers in the cord or lower part of the brain, so that the first hint of a loss of balance starts certain muscles contracting in just the right way to keep the body upright on the wheel.
Inhibitions.—An enormous number of reflex actions are possible in the body. Some of them produce opposite effects; that is, one reflex action may change or stop the action of another reflex. A sneeze is an example of an ordinary reflex action. Often, however, when you feel that you are going to sneeze, you can stop it by pressing your finger hard on your upper lip. This pressure on the lip produces a nerve impulse that inhibits, or stops, the sneeze reflex. A nerve message which tends to stop the action of another reflex is called an inhibitory impulse.
Involuntary Actions.—Many actions of the body are quite outside the control of the will. We cannot stop the heart from beating or make saliva flow by willing to do so.
The motor fibers which cause these actions are set in activity either by a reflex impulse from a sensory fiber, or perhaps by a more direct effect of some chemical or physical change in the tissues (producing what are called automatic actions).
It is probable that the actions of such creatures as insects are of this reflex and automatic kind, even the very complicated reflexes that are called instincts. It is instinct, for example, which teaches the butterfly to lay its eggs on the particular kind of plant that furnishes food for the cater-pillar that is to hatch from the eggs.
Voluntary Actions.—In some of our actions the will plays a definite part, although the line between voluntary and involuntary actions cannot always be sharply drawn. Certain activities may sometimes be unconscious reflexes and sometimes voluntary acts. The eye winks without our thinking about it, when something comes near it, but we can perform the same act by willing to do so. Inhibitions play a large part in voluntary actions. If something hot is put into the palm of your hand, there is a natural reflex impulse to drop it; but if it is not too hot, you can control the impulse and hold the object by making up your mind to do so.
The voluntary acts (done because we definitely will to do them) are all controlled by the forebrain, or cerebrum. The power of speech and of writing, and our more complicated actions, as well as consciousness—the knowledge of ourselves and our feelings and of the world about us—are all dependent upon the cerebrum and are lost if this part of the brain is injured. The simpler reflexes—such as the movements of balance or the tendency to draw back a part of the body when it is hurt—can be carried on by the lower part of the brain and the cord even when the cerebrum has been destroyed.
Habits and Their Importance.—The example of learning to ride a bicycle illustrates the importance of habit in daily life. Every time a certain act is performed, it becomes easier to do that thing again. The gray matter in the brain makes any nerve connection more readily the second time. It would probably be hard for you to keep tossing up and catching one ball a great many times without missing it. The jugglers in a circus, however, can keep half a dozen balls in the air without dropping one, simply because they have practiced until they have trained their reflexes so that eye and hand work perfectly together.
It is not only in juggling balls, and in other physical actions, that the forming of habits is important. There
are mental habits, too. The training of a soldier consists not only in teaching him how to use his gun, but also in teaching him to be brave and obedient and to do what he is told by his officer without a second's delay. The qualities of courage and obedience, which are needed among soldiers, are just as necessary in everyday life. People do not live in a world by themselves, like Robinson Crusoe. We must all work together, and if we are to be useful and lovable and are to get on in the world, we must learn to do quickly what we are
told by those in authority. Obedience is a habit which can be learned only by obeying. Cheerfulness and politeness are habits which become easier every time we practice them. Courage is a habit. Each time we master a difficult lesson, or perform an unpleasant task for the sake of some one else, we have blazed a trail that will make the way smoother next time.
Unfortunately the same thing holds true of bad habits.
Some people unconsciously allow themselves to form habits of disobedience, surliness, and cowardice which become so fixed that they can scarcely be overcome.
It may help us to realize the importance of doing the right thing, if we remember that every act and choice of ours really makes a kind of mark upon our nervous system which influences our future acts and choices. The time to form habits is in youth, when the nervous system and its reflexes are most easily influenced. You can bend a young sapling or even tie it in . knot; but when it has grown into a tree, nothing can change its form, except breaking it to bits. It is somewhat so with ourselves. A child can form habits easily, but a man who did not form good habits in youth has hard work to change afterward.
Hygiene of the Nervous System.—The nervous system, like the other parts of the body, needs rest to keep it healthy; and since the nerve cells of the cerebrum are working very hard all the time we are awake, their need for rest is particularly great. Fatigue after great physical or mental labor is caused by the fact that the muscles or the nerves have been worked so hard that waste products have collected too fast to be carried off by the blood and lymph, and are poisoning the tissues which produced them. Fatigue is nature's danger signal and should always be heeded. In the hurry and pressure of modern life, many people forget this and drive their bodies so hard that they break down and have nervous prostration or some other mental disease.
It is not hard work that causes these breakdowns so much as long-continued work and worry. A healthy body and brain will stand hard work. If a man tries to hold a heavy weight in the air too long, however, he may strain himself seriously; and it is much the same with the mind.
Long concentrated work of one kind is particularly harmful. If people were wiser, they would realize that they can accomplish more and keep in better health by working for shorter periods and resting between. Some kinds of amusements, like social gayeties, may be carried so far as to be more tiring than work itself; and many girls and women break down from working too hard at this kind of play.
When we do work, however, it is well to work seriously. Children, and grown people too, waste an enormous amount of time by half-working, letting their minds wander while they gaze out of the window. The habit of concentrating the mind can be steadily strengthened by practice, and it is one of the finest habits that any boy or girl can form.
Worry and crossness are very wearing, and one fit of anger harms the nervous system more than hours of hard work. It is important to form the habit of even-tempered cheerfulness, and this is possible for every one in a considerable degree. A cheerful person can do far more work and feel less tired than a cross and worrying one.
Rest and Play.—There are many ways of resting the brain and nerves; and people differ greatly in their rest needs. Some are rested by a change of work. In general, any occupation which calls for varied kinds of mental activity is much less of a strain than doing the same thing all the time. Many people get their rest or recreation (a very good word, since it means re-creation or building up again) by turning to some hobby, such as gardening, or studying birds, or collecting stamps or insects. These hobbies keep the mind active and interested, but call for the use of different brain cells from those which are used in daily work. The habit of reading is a great help to many people, who always turn to a book for relaxation. Others who do much brain work like to rest the nervous system by vigorous physical exercise, such as tennis, or by long walks in the open air; and some people's ideal of rest is to lie in a hammock or under an apple tree.
The Importance of Sleep.—The only complete rest for both body and mind is in sleep, and no one can keep healthy without satisfying this need. In sleep, the blood supply going to the brain is so decreased that we lose consciousness; that is, we do not think or feel or have any knowledge of what is going on about us. The depth or completeness of sleep varies. During the first part of the night one usually sleeps soundly, while toward morning it becomes more and more easy to wake up to full consciousness. Halfway between sleep and waking is the land of dreams in which, as the blood goes back into the brain and the nerve cells begin to act again, strange memories and associations of all kinds form a curious, confused picture in the mind. In a state of deep and dreamless sleep, the cells of the nervous system are best able to free themselves of their waste products and build themselves up again for the work of the coming day.
Individuals vary in the amount of sleep they require, al-though most grown people need about eight hours. Children who are growing fast and leading an active life need more, ten hours at least, and babies sleep from fifteen to twenty hours out of the twenty-four. It never pays to re-duce the hours needed for sleep, except now and then for some special emergency.
When one has been working very hard, or is "run down" by illness, or for any other reason needs special care, it is a great help to take a nap in the middle of the day, or even to lie down and rest a half hour without actually going tp sleep. A little rest which breaks the day's work is more useful than a much longer time added to the night's sleep.
QUESTIONS FOR DISCUSSION AND REVIEW
1. How does the nervous system enable the various organs to work together?
2. Give examples of how the work of the skin, the lungs, and the intestines is varied to meet different outside conditions.
3. What do we mean by the central nervous system? The sympathetic system? What is the work of each?
4. What are nerve fibers? Nerve cells? Where are each located?
5. What is the difference between a sensory nerve fiber and a motor nerve fiber? Compare with telephone wires.
6. Which are better protected, the brain and spinal cord, or the nerve fibers?
7. How does the size of a man's brain compare with that of other animals? Is a man's brain smoother or does it have more ridges than the brains of other animals?
8. What is the gray matter in the central nervous system? Where is it located?
9. Describe the brain and tell the special work of the different parts.
10. What is a reflex action? Give examples.
11. What is the use of reflex actions?
12. Some reflexes do not have to be learned (we say they are "instinctive"); others have to be acquired by practice. Give examples of each kind of reflex.
13. Under what control are the feet when we are learning to skate? After we have learned?
14. Why is it that after learning to play the piano you do not have to stop to think what notes to strike?
15. Are all reflexes useful? Name some that are harmful. How may they be controlled?
16. What do we mean by inhibitions? Give examples of inhibition which would make you a better citizen.
17. How do inhibitions play a part in learning to be brave? In learning a fire drill?
18. Can the process of digestion be controlled by the will? Can perspiration be controlled? Breathing?
19. Can habits of courage be acquired? Is telling the truth a habit?
20. Why is it important not to allow yourself to say "ain't" or "haven't got," even in fun?
21. Apply to some definite case these rules for habit formation:
1. When you have decided what to do, do it at the first opportunity that arises.
2. Repeat it as often as possible.
3. Do not let a single exception occur.
22. What is fatigue? Does it occur in muscles? In nerves?
23. Cheerfulness makes a smoothly running machine. Crossness and worry take the oil out of the joints and bearings. Under which of these conditions does the machine do better work?
24. How do nerves rest?
25. "All work and no play makes jack a dull boy." Why is play beneficial?
26. Why is sleep necessary? Make a set of rules for sleep, including number of hours, air, bed, and bedding.
27. What is the use of a nap in the middle of the day?