The Brain. We are exceedingly proud of our brain and inclined to regard it as the most important part of our body. So it is, in a sense; for it is the part which, through its connecting wires, called the nerves, ties together all the widely separated organs and regions in our body, and helps them to work in harmony with one another. We speak of it as the master and controller of the body; but this is only partially true.

The brain is not so much the President of our Cell Republic as a great central telephone exchange, where messages from all over the body are received, sifted, and transmitted in more or less modified form, to other parts of the body. Three-fourths of the work of the brain consists in acting as "middle-man," or transmitter, of messages from one part of the body to another. In fact, the brain is far more the servant of the body than its ruler; and depends for its food supply, its protection, its health, and its very life, upon the rest of the body. The best way to keep the brain clear and vigorous is to keep the muscles of the stomach, the liver, the heart, and the entire body in good health.

What the Brain Does. The brain is the very wonderful organ with which we do what we are pleased to call our thinking, and also a number of other more important things of which we are not conscious at all. It is a large organ, weighing nearly three pounds when full grown. In shape it is like an oval loaf of bread split lengthwise by a great groove down the centre, and with a curiously wrinkled or folded surface. The two halves of the brain, called hemispheres (though more nearly the shape of a coffee-bean), are alike; and each one, by some curious twist, or freak, of nature, receives messages from, and controls, the opposite half of the body—the right half controlling the left side of the body, while the left half controls the right side of the body. Thus an injury or a hemorrhage on the left side of the brain will produce paralysis of the right side, which is the side on which a stroke of paralysis most commonly occurs.

All the nerve fibres in each half or hemisphere of the upper brain run downward and inward like the sticks of a fan, to meet in a strap-like band, or stalk, which connects it with the base of the brain and the spinal cord. A very small amount of damage at this central part, or base, of the brain will produce a very large amount of paralysis. We may have large pieces of the bones of the skull driven into the outer surface of the brain, or considerable masses of our upper brain removed, or destroyed by tumors or disease, without very serious injury. But any disease or injury which falls upon the base of the brain, where these stalks run and big nerve-knots (ganglia) lie, will cause very serious damage, and often death.

The whole upper brain is a department of superintendence, which has grown up from the lower brain to receive messages, compare them with each other, and with the records of previous messages which it has stored up, thus giving us the powers which we call memory, judgment, and thought. Unfortunately, however, long and carefully as we have studied the brain, we really know little about the way in which it carries out these most important processes of memory, of judgment, and of thought, or even of the particular parts of it in which each of these is carried out.

No part of the brain, for instance, seems to be specially devoted to, or concerned in, memory or reason or imagination, still less to any of the emotions, such as anger, joy, jealousy or fear; so all those systems which pretend to tell anything about our mental powers and our dispositions by feeling the shapes of our heads, or the bumps on them, are pure nonsense.

The most important and highest part of the brain is its surface, a thin layer of gray nerve-stuff, often spoken of as the gray matter (the cortex, or "bark"), which is thrown into curious folds, or wrinkles, called convolutions. This gray matter is found in the parts of the nervous system where the most important and delicate work is done. The rest of the nervous system is made up of what is called white matter, from its lighter color; and this is chiefly mere bundles of telephone wires carrying messages from one piece of gray matter to another, or to the muscles.

We also know that a certain rather small strip of the upper brain-surface, or cortex, about the size of two fingers, running upward and backward from just above the ear, controls the movements of the different parts of the body. One little patch of it for the hand, another for the wrist, another for the arm, another for the shoulder, another for the foot, and so on. We can even pick out the little patch which controls so small a part of the body as the thumb or the eyelids. So when we have a tumor of the brain or an injury to the skull in this region, we can tell, by noticing what groups of muscles are paralyzed, almost exactly where that injury or tumor is. Then we can drill a hole in the skull directly over it and remove the tumor, lift up the splinter of bone, or tie the ruptured blood vessel.

Three other patches, or areas, running along the side of the brain, each of them about two inches across, are known to be the centres for smell, hearing, and sight, that for sight lying furthest back. Damage to one of these areas will make the individual more or less completely blind, or deaf, or deprived of the sense of smell, as the case may be.

At the lower part of the area which controls the muscles of the different parts of the body, above and a little in front of the tip of the ear, lies a very important centre, which controls the movements of the tongue and lips, and is known as the speech centre. If this should be injured or destroyed, the power of speech is entirely lost. This, curiously enough, lies upon the left side of the brain, and is the only one-sided centre in the body. Why this is so is somewhat puzzling, except that as speech is made up both of sound and of gesture, and our gestures are usually made with the right hand, it is not unreasonable to suppose that the speech centre should have grown up on that side of the brain which controls the right hand, which is, as you remember, the left hemisphere. What makes this more probable is that in persons who are "left-handed," the speech centre lies upon the opposite or right side of the brain. So it is waste of time and does more harm than good to try to "break" any child of left-handedness.

The Spinal Cord. Running downward from the base of the brain, like the stalk of a flower, is a great bundle of nerve-fibres, the central cable of our body telephone system, the spinal cord. This, you will remember, runs through a bony tube formed by the arches of the successive vertebrae; and as it runs down the body, like every other cable it gives off and receives branches connecting it with the different parts of the body through which it passes. These branches are given off in pairs, and run out through openings between the little sections of bone, or vertebrae, of which the spinal column is made up. They are called the spinal nerves, and each pair supplies the part of the body which lies near the place where it comes out of the cord.

The spinal nerves contain nerve wires of two sorts—the inward, or sensory, and the outward, or motor, nerves. The sensory, or ingoing, nerves come from the muscles and the skin and bring messages of heat and cold, of touch and pressure, of pain and comfort, to the spinal cord and brain. The outward, or motor, nerves running in the same bundle go to the muscles and end in curious little plates on the surface of the tiny muscle fibres, and carry messages from the spinal cord and brain, telling the muscles when and how to contract.

As the spinal cord runs down the body, it becomes gradually smaller, as more and more branches are given off, until finally, just below the small of the back and opposite the hip bones, it breaks up by dividing into a number of large branches which go to supply the hips and lower limbs.

While most of the spinal cord is made up of bundles of white fibres, carrying messages from the body to the brain, its central portion, or core, is made of gray matter. The reason for this is that many of the simpler messages from the surface of the body and the movements that they require are attended to by this gray matter, or ganglia, of the spinal cord without troubling the brain at all.

For instance, if you were sound asleep, and somebody were to tickle the sole of your bare foot very gently, the nerves of the skin would carry the message to the gray matter of the spinal cord, and it would promptly order the muscles of the leg to contract, and your foot would be drawn away from the tickling finger, without your brain taking any part in the matter, though, if you had been awake, you would of course have known what was going on.

This sort of reply to a stimulus, or "stirring up," without our knowing anything about it, is known as a reflex movement. Not only are many of these reflexes carried out without any help from the will, or brain, but they are so prompt and powerful that the brain, or will, can hardly stop them if it tries, as, for instance, in the case of tickling the feet. You can, if you make up your mind to it, prevent yourself from either wriggling, pulling your foot away, or giggling, when the sole of your foot is tickled; but if you happen to be at all "ticklish," it will take all the determination you have to do it, and some children are utterly unable to resist this impulse to squirm when tickled.

This extraordinary power of your reflexes has developed because only the promptest possible response, by jerking your hand away or jumping, will be quick enough to save your life in some accidents or emergencies, when it would take entirely too long to telephone up to the brain and get its decision before jumping. When you are badly frightened, you often jump first and discover that you are frightened afterwards; and this jump, under certain circumstances, may save your life. On the other hand, like all instinctive or impulsive movements, it may get you into more trouble than if you had kept still.

As you will see by the picture, the spinal nerves, which are given off from the cord in the lower part of the neck and between the shoulder blades, are gathered together into a great loose bundle to form the long nerve-wires needed to supply the shoulders and arms. Those given off from the small of the back just above the hips also run together to form, first a network and then a big single nerve-cord, called the sciatic nerve, which many of you have probably heard of from the frightfully painful disease due to an inflammation of it, called sciatica. It is the largest nerve-cord in the body, running down the middle of the back of the thigh to supply the muscles of two-thirds of the leg.[26]

The substance of both the spinal cord and the brain is made up of millions of delicate, tiny cells, called neurons, most of which, with very long branches, are arranged in chains for carrying messages, forming the white matter; while the others lie in groups, or ganglia, for sorting and deciding upon messages, forming the gray matter.

Just at the top of the spinal cord, where it passes into the skull and joins with the brain, it swells out into a sort of knob, about the size of a queen olive or the head of a gold-headed cane, which is known as the medulla, or "pith." This is the most vital single part of the entire brain and nervous system; and the smallest direct injury to it will produce instant death, partly because all the messages which pass between the brain and the body have to go through it, and partly because in it are situated the centres which control breathing and the beat of the heart, and another quite important but less vital centre,—that for swallowing.

How Messages are Received and Sent. Now to learn how smoothly and beautifully this nerve telephone system of ours works, and how simple it really is, although it has such a large number of lines and so many telephones on each line, and such a large central exchange, let us see how it deals with a message from the outside world. Suppose you are running barefoot and step on a thorn. Instantly the tiny nerve bulbs in the skin of the sole of your foot are stimulated, or set in vibration, and they send these vibrations up the sciatic nerve, into and up the whole length of the spinal cord, through the medulla, which switches them over to the other side of the brain up through the brain stalk, and out to the part of the surface (cortex) of the brain which controls the movements of the foot. All this takes only a fraction of a second, but it is not until the message reaches the brain-surface that you feel pain. If you were to cut the sciatic nerve, or even tie a string tightly around it, you could prick or burn the sole of your foot as much as you pleased, and you would not feel any pain at all.

As soon as the surface of the brain has recognized the pain and where it comes from, it promptly sends a return message back down the same cable, though by different nerve-wires, to the muscles of the foot and leg, saying, "Jerk that foot away!" As a matter of fact, this message will arrive too late, for the centres in the spinal cord will already have attended to this part of the matter, often almost before you know that you are hurt.

However, there is plenty of other work for the brain to do; and its next step, quicker than you can think, is to wake up a dozen muscles all over the body with the order, "Sit down!" And you promptly sit down. At the same time, the brain "central" has ordered the muscles of your arms and hands to reach down and pick up the foot, partly to protect it from any further scratch, and partly to pull the thorn out of it. Next it rushes a hurry call to the muscles controlling your lungs and throat, and says, "Howl!" and you howl accordingly. Another jab at the switchboard, and the eyes are called up and ordered to weep, while at the same time the muscles of the trunk of your body are set in rhythmic movement by another message, and you rock yourself backward and forward.

This weeping and rocking yourself backward and forward and nursing your foot seem rather foolish,—indeed you have perhaps often been told that they are both foolish and babyish,—but, as you say, you "can't help it," and there is a good reason for it. The howl is a call for help; and if the hurt were due to the bite of a wolf or a bear, or the cut had gone deep enough to open an artery, this dreadfully unmusical noise might be the means of saving your life; while the rocking backward and forward and jerking yourself about would also send a message that you needed help, supposing you were so badly hurt that you couldn't call out, to anyone who happened to be within sight of you. So that it isn't entirely babyish and foolish to howl and squirm about when you are hurt—though it is manly to keep both within reasonable limits.

If the message about the thorn had been brought by your eyes,—in other words, if you had seen it before you stepped on it,—then a similar but much simpler and less painful reflex would have been carried out. The image of the thorn would fall on the retina of the eye and through its optic nerve the message would be flashed to the brain: "There is something slim and sharp in the path,—looks like a thorn." When this message reached the brain, and not till then, would you see the thorn, just as in the case of the pain message from the foot. Then the brain would take charge of the situation just as before, flashing a hasty message to the muscles of the legs, saying, "Jump!" while its message to the throat and lungs, instead of "Yell," would be merely, "Say, 'Goodness!' or 'Whew!'" and you would say it and run on.

If the thing in the grass, instead of a thorn, happened to be a snake, and you heard it rustle, then the warning message would come through your ears to the brain, and you would jump just the same; though, as it is not so easy to tell by a hearing message exactly where the sound is coming from, you might possibly jump in the wrong direction and land on top of the danger.

This is the way in which you see, hear, and form ideas of things. Your eye telegraphs to the brain the colors; your ear, the sounds; and your nose, the smells of the particular object; and then your brain puts these all together and compares them with its records of things that it has seen before, which looked, or sounded, or smelt like that, and decides what it is; and you say you see an apple, or you hear a rooster crow, or you smell pies baking. Remember that, strange as it may seem, you don't see an orange, for instance, but only a circular patch of yellowness, which, when you had seen it before, and felt of it with your hand, you found to be associated with a feeling of roundness and solidness; and when you lifted it toward your nose, with the well-known smell of orange-peel; so you called it an orange. If the yellow patch were hard, instead of elastic, to the touch, and didn't have any aromatic smell when you brought it up to your nose, you would probably say it was a gourd, or an apple, or perhaps a yellow croquet ball. This is the way in which, we say, our senses may "deceive" us, and is one of the reasons why three different people who have seen something happen will often differ so much in their accounts of it.

It is not so much that our senses deceive us, but that we draw the wrong conclusions from the sights, sounds, and smells that they report to our brains, usually from being in too great a hurry and not looking carefully enough, or not waiting to check up what we see by touching, hearing, or tasting the thing that we look at.

This message-and-answer system runs all through our body. For instance, if we run fast, then the muscle cells in our legs burn up a good deal of sugar-fuel, and throw the waste gas, or smoke, into the blood. This is pumped by the heart all over the body, in a few seconds. When this carbon dioxid reaches the breathing centre in the medulla, it stirs it up to send promptly a message to the lungs to breathe faster and deeper, while, at the same time, it calls upon the circulation centre close to it, to stir up the heart and make it beat harder and faster, so as to give the muscles more blood to work with. If some poisonous or very irritating food is swallowed, as soon as it begins to hurt the cells lining the stomach, these promptly telegraph to the vomiting centre in the brain, we begin to feel "sick at the stomach," the brain sends the necessary directions to the great muscles of the abdomen and the diaphragm, they squeeze down upon the stomach, and its contents are promptly pumped back up the gullet and out through the mouth, thus throwing up the poisons.

And so on all over the body—every tiniest region or organ in the body, every square inch of the skin, has its special wire connecting it with the great telephone exchange, enabling it to report danger, and to call for help or assistance the moment it needs it.