CHAPTER I.

THE NERVOUS SYSTEM AND SOME OF ITS MORE IMPORTANT FUNCTIONS.

The nervous system is made up of a nerve centre and nerves.

The great nerve centre is the _Brain_ and _Spinal Cord_.

The brain is a body weighing about forty ounces, and fills a cavity in the upper part of the skull. The spinal cord, commonly called spinal marrow, is directly connected with the brain. The skull rests upon the spinal column, or backbone, and there is a cavity inside the whole length of this column, which contains the cord. There is an opening through the base of the skull where it rests upon the spinal column, and it is through this opening that the fibres of the cord go, to pass into and become a part of the brain. These most important parts are carefully protected by a strong bony covering.

Many nerves are given off from the brain and cord and go practically everywhere, so that every part of the body is supplied with them. These nerves are white cords of different sizes; the largest nerve of the body, the one that goes to the leg, called the sciatic, is as large as the little finger.

There are really two brains and two cords, as along the central line of the body there is a division of the brain and cord, making two halves exactly alike. These halves are connected together, the division not being complete.

Nerves are given off in pairs; for example, from either side of the brain arises a nerve that goes to each eye. So two nerves exactly alike spring from the two sides of the spinal cord, going to each arm.

A nerve is composed of a bundle of fibres, microscopic in size. As a nerve passes to the extremities it divides by branching much as does an artery, and thus a bundle of fibres is distributed to a muscle, or a part of the skin, or to an organ, and every part of the body has a direct nerve supply, much as you saw in the microscope it was supplied with blood by means of the capillaries. We cannot prick our finger with the finest needle but nerve, fibres are irritated, and we feel it, and capillaries are injured and we get a drop of blood.

Most of the nerves that go to the arms, legs, and organs of the chest and abdomen, arise in and proceed from the spinal cord, but some of the fibres begin in the brain and are continued down the cord, where, joining with fibres that originate in the cord itself, both go to make up the nerve, thus connecting all parts of the body with the great centre.

The brain and cord are made up of blood-vessels, nerve cells, nerve fibres, and, holding them all together, connective tissue. The cells are very small, being microscopic in size; there are an immense number of them, and they make up most of the gray matter or outside of the brain, but in the spinal cord the gray matter is in the centre. The fibres that go to make up the nerves begin and spring from the cells, and they also unite them together.

The cells are gathered into groups, which have each a separate function to perform. There is a group from which the nerve of the eye proceeds; another for the nerve that goes to the ear; another for the nerve that goes to the arm; and another for the nerve of the heart. There is a group that presides over speech, and other groups that preside over mental action, while all of these are connected together by fibres. Thus it appears that the brain is a true "centre," and the nerves but the means of connection between different parts of the body and the brain, and also between different parts of the brain.

Nerves have two special functions: one to carry impressions made upon the fibres, that end in the different parts and organs of the body, to the brain; another to carry from the nerve cells so-called "nerve impulses," to the different parts and organs of the body. Some nerves have in themselves these two functions, as the nerves that go to the arm or leg; others have but one, as the optic or eye nerve, which can only carry the sensation of sight from the eye to the brain.

The nerves that carry sensations to the brain are called _Sensory Nerves_. The nerves that carry motor impulses from the brain are called _Motor Nerves_.

There are five special organs of sense, each receiving different impressions, and sending by its sensory nerve or nerves a different character of sensation to the brain, namely:

The eye, giving sensations of light and color.

The ear, giving sensations of sound.

The nose, giving sensations of smell.

The mouth, giving sensations of taste.

The skin, giving sensations of touch, with ideas of roughness, smoothness, hardness, softness, heat, and cold.

There must be, in every case, a direct nerve connection from the organ of special sense to the special group of cells in the brain to which the nerve goes. If the connection is broken at any point, the impression made upon the fibres in the organ of sense cannot reach the brain. Only after the impression reaches the brain and the cells are affected, do we become conscious of a sensation. We then say, as the case may be, I see, or hear, or smell, or taste, or feel something.

It thus appears that these organs of sense simply receive the impressions made upon them to transmit to the brain, and it is really the brain that sees, hears, smells, tastes, and feels. By the action of the organs and nerves of special sense we get all our knowledge of the external world, and, probably, if we had no organs of sense, we would have no consciousness of our existence.

Pain is due to abnormal action of sensory nerves, caused by disease, injury, or pressure, and the irritation made, being carried to the brain makes us conscious of the peculiar sensation we call pain. So the want of food or water makes an impression upon nerves, which being carried to the brain causes a peculiar sensation, and we say we feel hungry or thirsty.

The _Motor Nerves_ arise in the cells of the brain and cord. Those which go to the voluntary muscles cause them to contract, and are under the control of the will. If the cells are diseased, if they do not get enough arterial blood, or are poisoned by carbonic acid, or if the nerves are diseased, injured, or cut, so that nerve impulses cannot be sent from the brain to the muscles, we have paralysis of a muscle or a group of muscles, according to the extent of the injury. Now we can appreciate the force of this teaching in the physiology of the muscular system, that "paralysis is a loss of power, either partial or complete, to contract muscles, due to disease of the nerves."

By the ready action of our mind, the quick working of our will, we direct and control the action of our muscles, so as to perform with the utmost skill and ease the varied and innumerable movements of our body.

It seems very easy to do this, but watch a child learning to walk; it is educating its mind and will to control the muscles, and it is a slow and difficult education.

But all motor impulses and bodily activities are not under the control of the will. The heart is supplied with motor nerves, but we cannot by our will stop its beating or control its action. The taking of food makes a mental impression, and without the will being involved, impulses are sent to the glands of the mouth, setting them actively at work, and saliva flows. So the stomach begins to churn food when it is introduced, and the liver is kept at work making bile and sugar, and we breathe when we are asleep.

All the organs of the body are supplied with motor nerves, that regulate their action and give them the power to do their function or work, but with the exception of the muscles, this power is sent without the action of the will.

Our brains are very busy. While we are awake we are constantly receiving sensations, we are thinking, remembering, willing, and sending many messages every minute, and directing power to all parts of the body. The brain works and gets tired, just as the rest of the body gets tired, and, if abused, injured, or overworked, may become diseased as may any part. Its tissues wear out, are burned up, and require the same supply of material to repair them that any other part of the body requires. It needs then rest, good food, good blood, and plenty of oxygen.

No wonder some brains give out, and fail to do their work properly, and so cause insanity.