service the clearer will the impression be; and that the clearer the impression is, the longer will it stay by us.

Along with all these facts it is important to remember that each separate sense depends on the work done by three parts of a delicate piece of machinery:

1. Apparatus which receives stimulus — eye, ear, nose, skin, etc.

2. Axons which carry the impulse.

3. Cell bodies in the cortex which recognize the impulse when it arrives.

In the case of each sense, also, we must suppose that the outside apparatus itself knows no more about what is happening to it than the mouthpiece of a telephone knows what we say when we speak into it. In point of fact, the receiving apparatus of each sense is nothing 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 axons, we say we have heard something. Eye apparatus receives a stimulus, and when that stimulus has reached the brain on eye axons, 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 axons. 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 neurons 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 seeing part 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 region that smells, 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 the human brain, however, affairs are generally better balanced. Each sense has been active, and all

are fairly well developed. But perhaps we wonder what happens in a case like that of Laura Bridgman. Did her brain betray anything about its lacking senses?

Doctors not only asked precisely this question, but they answered it. They could do this because they were granted the privilege of examining Laura Bridgman's brain after she died. The result of the examination was that they found the cortex thinnest at the centers for seeing, hearing, tasting, and smelling.

Not only this, but they also saw that the part which belonged to the left eye-the eye that saw nothing from the time Laura was two years old-was much thinner than the part which belonged to the right eye. As we know, this right eye had served her a little until she was eight years old.

As might have been expected, the touch region of Laura's brain was found to be wonderfully developed. In view of these facts, 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 it is ruined by disease, accident, or careless use, no amount of striving on our part will restore it to us. (Look up Good Health on the care of eye and ear.)

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.