Page:Popular Science Monthly Volume 25.djvu/625

Rh to one another has a more practicable object than the mere gratification of scientific curiosity; it is a knowledge upon which the happiness and prosperity, or the reverse, of millions of our fellow-creatures may depend.



F you examine the brain of a dog, or an ape, or a man, you will see that it is made up of two kinds of substance, gray and white. The gray substance, which is formed of round bodies of nervous matter called nerve-cells, is spread out in a thin layer over the entire surface of the brain. The white substance constitutes the center and body of the organ, and consists of white threads or nerve-fibers which pass in various directions through the brain and end in the cells of the gray matter. It is the office of the white fibers to convey messages; it is the office of the gray cells to dispatch them, or to receive and register them.

If a brain be properly torn apart, it can be shown that many of the white threads are collected into bundles. These bundles, each of which contains many thousand threads, can be separated from one another and followed to their terminations. It will then be found that each bundle, or tract, as it is called, connects some one organ of the body with some one region of the gray matter on the surface of the brain. For example, one tract joins the muscles of one half of the body with the lateral part of the opposite half of the brain; another ascends from the surface of the body, being made up of many fibers, each of which comes from one little area of skin, and this tract ends in the surface of the brain just behind the first one; another bundle comes from the eye and goes to the posterior part of the brain. So too the ear, the nose, the tongue, send in their bundles, and each of these goes to a definite and separate region of the surface. And thus, as every part of the body is connected by its own tract with its own part of the gray matter, we can imagine upon the surface of the brain a map of the entire body laid out, and can say, as Meynert does, that the surface of the body is projected upon the surface of the brain.

Each of the little white threads, like an electric wire in a cable, is insulated from every other by a sheath. It is therefore impossible for a message sent from one end of the thread to leave it; the message must go to the other end of the thread. Therefore, an irritation set up in any organ of the body is always transmitted to that part of the brain with which the organ is joined, and can not reach any other part directly, although it may do so indirectly, by means of association fibers which join the different regions with one another. The 