The Nervous System : Part 1

Hygienic Physiology: with Special Reference to the Use of Alcoholic Drinks and Narcotics
by Joel Dorman Steele, Ph.D.

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"Mark then the cloven sphere that holds All thoughts in its mysterious folds, That feels sensation's faintest thrill, And flashes forth the sovereign will; Think on the stormy world that dwells Lock'd in its dim and clustering cells; The lightning gleams of power it sheds Along its hollow, glassy threads!"

"As a king sits high above his subjects upon his throne, and from it spokes behests that all obey, so from the throne of the brain cells is all the kingdom of a man directed, controlled, and influenced. For this occupant, the eyes watch, the ears hear, the tongue tastes, the nostrils smell, the skin feels. For it, language is exhausted of its treasures, and life of its experience; locomotion is accomplished, and quiet insured. When it wills, body and spirit are goaded like overdriven horses. When it allows, rest and sleep may come for recuperation. In short, the slightest penetration may not fail to perceive that all other parts obey this part, and are but ministers to its necessities."

- Odd Hours of a Physician.

Structure. - The nervous system includes the brain, the spinal cord, and the nerves. It is composed of two kinds of matter - the white, and the gray. The former consists of minute, milk-white, glistening fibers, sometimes as small as 1/25000 of an inch in diameter; the latter is made up of small, ashen-colored cells, forming a pulp-like substance of the consistency of blancmange. This is often gathered in little masses, termed ganglions, because, when a nerve passes through a group of the cells, they give it the appearance of a knot. The nerve fibers are conductors, while the gray cells are generators, of nervous force. The ganglia, or nervous centers, answer to the stations along a telegraphic line, where messages are received and transmitted, and the fibers correspond to the wires that communicate between different parts.

The BRAIN is the seat of the mind. Its average weight is about fifty ounces. It is egg-shaped, and, soft and yielding, fills closely the cavity of the skull. It reposes securely on a water bed, being surrounded by a double membrane, delicate as a spider's web, which forms a closed sac filled, like the spaces in the brain itself, with a liquid resembling water. Within this, and closely investing the brain, is a fine tissue, with a mesh of blood vessels which dips down into the hollows, and bathes them so copiously that it uses one fifth of the entire circulation of the body. Around the whole is wrapped a tough membrane, which lines the bony box of the skull, and separates the various parts of the organ by strong partitions. The brain consists of two parts - the cerebrum, and the cerebellum.

The Cerebrum fills the front and upper part of the skull, and comprises about seven eighths of the entire weight of the brain. As animals rise in the scale of life, this higher part makes its appearance. It is a mass of white fibers, with cells of gray matter sprinkled on the outside, or lodged here and there in ganglia. It is so curiously wrinkled and folded as strikingly to resemble the meat of an English walnut. This structure gives a large surface for the gray matter, - sometimes as much as six hundred and seventy square inches. The convolutions are not noticeable in an infant, but increase with the growth of the mind, their depth and intricacy being characteristic of high mental power.

The cerebrum is divided into two hemispheres, connected beneath by fibers of white matter. Therefore we have two brains, as well as two hands and two eyes. This provides us with a surplus of brains, as it were, which can be drawn upon in an emergency. A large part of one hemisphere has been destroyed without particularly injuring the mental powers, - just as a person has been blind in one eye for a long time without having discovered his loss. The cerebrum is the center of intelligence and thought. Persons in whom it is seriously injured or diseased often become unable to converse intelligently, both from inability to remember words and from loss of power to articulate them.

The Cerebellum lies below the cerebrum, and in the back part of the head. It is about the size of a small fist. Its structure is similar to that of the brain proper, but instead of convolutions it has parallel ridges, which, letting the gray matter down deeply into the white matter within, give it a peculiar appearance, called the arbor vite, or tree of life. This part of the brain is the center for the control of the voluntary muscles, particularly those of locomotion. Persons in whom it is injured or diseased walk with tottering and uncertain movements as if intoxicated, and can not perform any orderly work.

The Spinal Cord occupies the cavity of the backbone. It is protected by the same membranes as the brain, but, unlike it, the white matter is on the outside, and the gray matter is within. Deep fissures separate it into halves, which are, however, joined by a bridge of the same substance. Just as it starts from the brain, there is an expansion called the medulla oblongata.

The Nerves are glistening, silvery threads, composed, like the spinal cord, of white matter without and gray within. They ramify to all parts of the body. Often they are very near each other, yet are perfectly distinct, each conveying its own impression. Those which carry the orders of the mind to the different organs are called the motory nerves; while those which bring back impressions which they receive are styled sensory nerves. If the sensory nerve leading to any part be cut, all sensation in that spot will be lost, while motion will remain; if the motory nerve be cut, all motion will be destroyed, while sensation will exist as before.

Transfer of Pain. - Strictly speaking, pain is not in any organ, but in the mind, since only that can feel. When any nerve brings news to the brain of an injury, the mind refers the pain to the end of the nerve. A familiar illustration is seen in the "funny bone" behind the elbow. Here the nerve (ulnar) gives sensation to the third and fourth fingers, in which, if this bone be struck, the pain will seem to be. Long after a limb has been amputated, pain will be felt in it, as if it still formed a part of the body - any injury in the stump being referred to the point to which the nerve formerly led.

The nerves are divided into three general classes - the spinal, the cranial, and the sympathetic.

The Spinal Nerves, of which there are thirty-one pairs, issue from the spinal cord through apertures provided for them in the backbone. Each nerve arises by two roots; the anterior is the motory, and the posterior the sensory one. The posterior alone connects directly with the gray matter of the cord, and has a small ganglion of gray matter of its own at a little distance from its origin. These roots soon unite, i. e., are bound up in one sheath, though they preserve their special functions. When the posterior root of a nerve is cut, the animal loses the power of feeling, and when the anterior root is cut, that of motion.

The Cranial Nerves, twelve pairs in number, spring from the lower part of the brain and the medulla oblongata.

1. The olfactory, or first pair of nerves, ramify through the nostrils, and are the nerves of smell.

2. The optic, or second pair of nerves, pass to the eyeballs, and are the nerves of vision.

3, 4, 6. The motores oculi (eye movers) are three pairs of nerves used to move the eyes.

5. The trifacial, or fifth pair of nerves, divide each into three branches - hence the name - the first to the upper part of the face, eyes, and nose; the second to the upper jaw and teeth; the third to the lower jaw and the mouth, where it forms the nerve of taste. These nerves are implicated when we have the toothache or neuralgia.

7. The facial, or seventh pair of nerves, are distributed over the face, and give it expression.

8. The auditory, or eighth pair of nerves, go to the ears, and are the nerves of hearing.

9. The glos-so-pha-ryn'-ge-al, or ninth pair of nerves, are distributed over the mucous membrane of the pharynx, tonsils, etc.

10. The pneu-mo-gas'-tric, or tenth pair of nerves, preside over the larynx, lungs, liver, stomach, and one branch extends to the heart. This is the only nerve which goes so far from the head.

11. The accessory, or eleventh pair of nerves, rise from the spinal cord, run up to the medulla oblongata, and thence leave the skull at the same opening with the ninth and tenth pairs. They regulate the vocal movements of the larynx.

12. The hyp-o-glos'-sal, or twelfth pair of nerves, give motion to the tongue.