Page:Popular Science Monthly Volume 25.djvu/627

Rh may be, he no longer looks for his food and water in their usual places, and when they are put before him he does not seem to know them as food and water until his nose is put into them, when he recognizes them by other senses than sight. The sight of the whip, which used to make him run into a corner, does not frighten him any more, though he jumps when he hears it snap. He used to give his paw when the hand was held out for it. Now he will not do so until the word paw is spoken, when he holds it up as before. The dog is not blind, but he has lost the power of recognizing objects formerly recognized by sight. He has been deprived, by the operation, of his sight-memory pictures, or sight-imaging power. He has been put back, as far as one sense is concerned, into the condition in which he was when born—that is, destitute of knowledge acquired by sight-perception. He acts just like a puppy; for he soon begins to smell and lick objects in an inquiring way, and to run to and examine curiously things with which he was formerly familiar. He sees these things, he learns again to know them; in a word, he begins at once to lay in a new store of memory-pictures. It is only necessary to put his nose into water a few times; after that he looks for and finds it when he is thirsty. Then he begins to know his master. The whip soon becomes again a dreaded object. And in the course of two or three months he has gained a new set of memories and recognizes objects just as before the operation.

In the first dog, the entire posterior part of the brain was removed, and the dog was made permanently blind. In the second dog, a portion of this part of the brain was cut out, and the dog was deprived of his sight-memory. He was, however, able to recover. And, if by successive operations the experiment be repeated on the same dog, it will be found that recovery is always possible until the entire posterior part of the brain is removed, when, like the first dog, he becomes permanently blind. The recovery then was possible because around the area cut out there was left a ring of gray matter which was in connection with the eye; and in this ring of gray matter, which formerly contained no memory-pictures, the new memory-pictures were stored. All the posterior part of the brain in the dog is, therefore, a potential area for sight-memories. The actual area of sight-memories occupies only a part of the potential area. If the actual area is cut out, but a part of the potential area remains, the dog is temporarily deprived of sight-memory, but can recover. If the potential area is entirely extirpated, the dog remains blind, and can never regain his memories. The distinction between actual and potential memory is important, as we shall see when we come to similar phenomena in man.

The experiments just described were first made by Hermann Munk, Professor of Physiology in the University of Berlin, and they have been confirmed by many other experimenters. What has thus been proved of the location of perception by sight, and of sight-memories in the posterior part of the brain, has also been proved of other senses