more than a marvelous device for receiving its own special kind of stimulus. Ear apparatus receives a stimulus, and when that stimulus reaches the brain by way of ear fibers we say we have heard something. Eye apparatus receives a stimulus, and when that stimulus has reached the brain on eye fibers we say we have seen something. Skin and nose and tongue serve us in the same way. Each is a piece of apparatus that receives stimulus of its own kind and sends it up to the brain on its own distinct set of fibers. In every case the brain is the receiving point; the cells up there feel our sensations for us.

Since we know these many and various facts, and since we also know that exercise always develops any part of the body that is used vigorously, we are not surprised to hear that by examining a brain after death a trained scientist can tell just which set of nerve cells did the most work during life.

These men may, for example, take a bird that has lost. its life, and point to a certain place on the brain. “You see it is very much enlarged," they say. "That is the part that always had the most exercise. It is the sight center of the cortex." And at once we call to mind the stories we have heard about the carrier pigeons and other birds, — about the keenness of their vision and the distance they can fly from home.

The brain of a dog may be examined next. "There!" the scientist exclaims, “do you see this part? It is the

center for smell, and it is always greatly enlarged in dogs." And now we recall all our dog stories. We remember that a bloodhound will trace a man through a crowded city, that the scent of a dog is one of his most remarkable points.

The examination might go on from brain to brain, from animal to animal, each showing that one of the senses was more highly developed than any of the others.

In human brains, however, affairs are generally better balanced, unless there has been some great affliction during life. This was true of Laura Bridgman. She was deaf and dumb and blind and had no sense of smell. Her one connection with the world was through her sense of touch. As a result, the nerve cells of touch received constant daily exercise, while the nerve cells of all the other senses received no exercise whatever. Then came the startling discovery; for after Laura Bridgman died her brain itself told the story of her senses. Doctors examined the cortex and found that it was thinnest at the centers of seeing, hearing, tasting, and smelling. More than this, as might have been expected, the doctors also found that the touch region of Laura's brain was wonderfully developed.' In view of all this we draw the following conclusions for immediate use.

1. Although the outside apparatus does nothing but receive stimulus of one sort or another, still, if

1 Much more is told about Laura Bridgman in Control of Body and Mind.

it is ruined by disease, accident, or careless use, no amount of striving on our part will restore it to us.1

2. If the apparatus of one sense has been wrecked, the other senses may be so highly developed as to help make up the loss.

3. Persistent exercise of any sense will increase the thickness of the part of the cortex to which it belongs.

Although no examination of the cortex of our own cerebrum is possible while we are alive, still we may have the comfort of knowing that we are improving its quality here or there in proportion as we are giving one sense or another more or less exercise. The truth is that our senses are our best friends or our worst enemies in just such measure as we train or neglect them.

1 Look up Good Health on the care of eye and ear.

CHAPTER XXXIX

HELP THROUGH HAPPINESS; OR, THE SYMPATHETIC

GANGLIA

Before studying this chapter, test yourself in two ways. First, try with all your might to make your heart stop beating. Try to prevent the great arteries from expanding and contracting as the blood surges through them in pulses. See whether, by thinking and willing hard enough, you can prevent your sweat glands and oil glands from manufacturing salt water and oil. Will your stomach obey you when you command it to stop digesting your

food?

Now turn the tables. Say to your heart as it pounds steadily along: "Beat faster. Beat faster. You must beat faster." Will it obey you? No; it goes neither faster nor slower by the fraction of a second. Your brain and your heart seem to be as independent of each other as if they belonged to different bodies and lived in different worlds.

Nevertheless, as we all know, life itself depends on the beating of the heart. We know that whenever it stops and fails to start again we shall die, but from year's end to year's end we think nothing about it. At night we lie down to sleep with no anxiety lest the steady