Brain

that part of the nervous system inclosed in the skull. It is continued downward as the spinal cord through an opening at the base of the skull, called the foramen magnum.

The cell substance of which the brain is built is exceedingly soft and easily injured by pressure or friction. It is well protected, however, not only by its thick, bony covering, the skull, but by three membranes which entirely cover it. The outermost of these membranes is thick and tough, and fits closely by its outer roughened surface to the inner surface of the bones. Its inner surface, next to the brain, is smooth. This outer membrane is still called by its early Latin name, dura mater, which means hard mother, because in early times it was thought that all membranes of the body originated from this. The second membrane is like a delicate transparent sac, its sides flattened close against each other. It fits against the smooth inner surface of the dura mater, and is called the arachnoid, or spider's web, because it is so soft and thin. Beneath the arachnoid is a third thin membrane known as the pia mater, which is Latin for kind or tender mother. This membrane is composed of a network of blood vessels held together by tissues and it is these blood vessels that supply the nourishment and remove the wastes of the entire brain. It is almost impossible to remove the pia mater from the brain, because it dips into every fissure and surrounds all parts.

On examining the brain as a whole we find that it consists of three parts: a large anterior part, known as the fore-brain or cerebrum; a narrow middle portion, the mid-brain; and a posterior part, the hind-brain. The fore-brain occupies the greater part of the skull-cavity, and is nine-tenths of the bulk of the entire brain. A deep fissure partially divides the fore-brain into right and left hemispheres. These two hemispheres are united near their under surface by a narrow band of fibers known as the corpus callosum, or callous body. The mid-brain is very small and narrow, and carries the optic lobes, two swellings, into which the sensory nerves from the eyes enter. The hind-brain consists of two parts. One of these is just posterior to and beneath the fore-brain, and is known as the cerebellum; the other is united at its narrowest part with the spinal cord. This is known as the medulla oblongata.

The structure of the brain is very complex, and only the main features can be considered here. In lower animals, such as the frog, the fore-brain, mid-, and hind-brain, lie in a horizontal plane; but in the human being, they are doubled on one another, so that they form a figure something like a question mark (?). The fore-brain occupies all of the upper curved portion of the question mark; the cerebellum is below in the upper part of the stem; and the medulla oblongata occupies the lowest part of the stem.

If the brain is viewed from the top, only the two hemispheres of the fore-brain are seen. Each of these is seen to be made up of folds of pale gray matter. These folds are numerous, and are arranged so as to form more or less definite divisions of the hemispheres, called lobes. The folds are known as convolutions; the indentations separating the lobes are known as fissures. Each convolution and each lobe bears a definite name, and it is found very useful in the study of normal, as well as diseased brain-activity, to know both the name and the location of these parts.

When the brain is viewed from below, we can see a large part of the underside of the fore-brain, as well as the mid- and hind-brain. Projecting from the fore-brain, on each side of the mid-brain, are the olfactory lobes. Nerves run through these lobes from the nose to the cerebrum, and are the means by which is gained a knowledge of the smell of substances. Just back of the olfactory lobes are the crossed optic or eye-nerves, which enter the mid-brain.

Back of the mid-brain is a band of white matter called the pons, or bridge, because it connects the two lobes of the cerebellum. The cerebellum has a wrinkled appearance; but the wrinkles are regularly arranged and really appear like very small convolutions. The surface of the medulla oblongata is smooth.

Twelve pairs of cranial nerves can be seen on the under side of the brain. Some of these nerves carry impulses from the brain to the various parts of the head or body, and are called motor nerves, or nerves that cause motion. The nerve which governs the movement of the eyeball is a motor nerve. Others carry impulses from the various parts of the body to the brain, and are called sensory nerves. The optic nerve is sensory. Still other nerves are both motor and sensory, as in the case of the fifth pair of cranial nerves, which carry impulses from the brain to the muscles of the jaws and eyelids, and impulses from the face and teeth to the brain. The tenth pair of cranial nerves is very important. It largely governs the action of the lungs, the heart, and the stomach. It is called the vagus, or wanderer.

The above are some of the features that can be noted by observation of the exterior of the human brain. When the brain is cut into halves, in the direction of the median fissure, one finds that it is a hollow organ, with its cavity surrounded by outgrowths or walls. All regions of the brain are connected with its cavity, which, in turn, is connected with the cavity of the spinal cord. During life this continuous cavity is filled with a fluid known as the cerebro-spinal fluid. Its walls are supplied with blood from a network of blood vessels, and it is the rupture of some of these which is one cause of apoplexy.

If we cut into the brain from the top downward we find that its solid parts consist of two kinds of matter, white and gray. In the cerebrum and the cerebellum, the gray matter, about one-eighth inch in thickness, is on the outside; the white forms the central core of the mass. In the medulla oblongata the gray matter is broken up into masses which serve as centers of origin for various nerves.

In the lower animals, as in the frog, the brain is essentially like that of the human; but the various parts are not developed to the same extent as in the human brain. Thus, in the frog, the cerebrum is comparatively small and not at all convoluted, but smooth; the mid-brain and the hind-brain are large. The higher the animal in the scale of life, the more complex is the structure of the brain, shown by wrinkling and folding of its surface into convolutions and lobes. Thus, the brain of the chimpanzee, of the horse, cow, and sheep, is more like the human brain than is that of the fish, frog, or bird.

It is thought that these differences in structure are somehow related to the amount and kind of work that can be done by the brain. In all animals the brain controls all the higher work or activities of the animal, such as voluntary movement and sensations. The brain also controls such important operations within the body as the beating of the heart, breathing, and digestion. An animal may have the stomach removed and continue to live; but it cannot survive the loss of the brain.

The results of experimentation indicate that the seat of intelligence, memory, will, and the emotions in man is in the gray matter or cortex of the cerebrum. The cortex is also the seat of conscious sensation. By this, we mean that if the gray matter of the cerebrum were removed, a human being would be unable to see, hear, taste, smell, or to exercise the sense of touch. A piece of ice might be placed in the hand and no feeling of cold would result. If the sun shone ever so gloriously upon one without a cerebral cortex, everything would seem to such an one in total darkness. Though breathing and other life processes might continue, though movement would be possible, there would be no consciousness of movement. Such a being would show no signs of fear or pleasure; there would be no feeling of hunger nor any desire to move or to work, to sleep, or to talk.

Patient, careful study of the brain has revealed many wonderful things; but much remains still unknown. We know that the brain is made up largely of nerve cells and nerve fibers. The fibers connect the brain with all parts of the brain with every other part. The power to think is connected in some way with the activity of the nerve cells; but in just what way, we do not know. Nerve cells seem to generate and receive impulses and sensations; how they do it, we do not know. The changes which take place in nerve cells are essentially like those which take place in muscle cells. Nerve cells require air, food, rest, and exercise for their continued well-being, as do other cells and tissues in the body. But they can perform a work in the body that no other cells seem able to do. In the same way we know that nerve fibers conduct impulses and sensations. How they do it, we do not know.-ELLEN TORELLE.