A Handbook of Health by Woods Hutchinson (learn to read books .txt) 📖

Diagram to show brain, spinal cord, and larger nerves.

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,