Page:Popular Science Monthly Volume 25.djvu/626

610 of the brain, thus studied, gives a clear indication that the different regions of its surface govern different organs of the body, and that each region has a distinct function to perform.

It is an admitted fact that an irritation set up at one end of a sensory nerve and sent to the brain produces a change of state in the gray cells which receive it. That change of state is known to us as the conscious perception of a sensation. The conscious perception does not occur in the organ irritated, nor in the nerve which carries the irritation. It occurs in the brain. The perception of an object seen does not take place in the eye, nor in the optic nerve, but in the posterior part of the brain where the tract from the eye terminates in gray cells. In like manner each sensation is consciously perceived in that part of the brain with which the sensory organ is connected whose irritation produced the sensation.

Being perceived, the sensation is in some way registered and preserved, so that when a second similar irritation is sent inward we not only perceive it, but recognize it as a matter of former experience. But, independently of a second perception, we have evidence that the first is preserved in the fact that we can call it up to consciousness by a voluntary effort, and make it, by means of memory, an object of thought. In both these processes the same part of the brain is in action which originally perceived the sensation. But, as sensations are perceived in various regions, it becomes evident that memories are stored up in various regions. If this is so, our various kinds of memory must be independent of each other, and one may be lost while others remain. We shall soon see that this is the fact.

If you lay bare the brain of a dog, and carefully cut out all the posterior part of both halves or hemispheres, you will find, when the dog recovers from the operation, that it is totally and permanently blind. It can smell, and hear, and taste. It can run about, and can perceive sensations of all kinds except those of sight. If from the brains of other dogs you cut out other parts, but leave the posterior part untouched, sight will not be affected in any case. These physiological experiments show that perceptions of sight occur in the posterior parts of the brain, the parts to which we have already traced the white threads from the eye.

If, instead of cutting out the whole of the posterior part of the brain, you select the central portion of the posterior part, leaving a ring of tissue about it uninjured, the result is more interesting. (See Fig. 1, A.) After a few days, when the wound is healed, you will find that the dog's hearing, smell, taste, motion, and general sensation are in no way affected. The animal runs about the room, and, unlike the first dog, either avoids or jumps over any obstacle which may be put in his way. He can therefore see the obstacle. But the sight of other dogs, or of men, whom he used to recognize with signs of pleasure or dislike, no longer affects him at all. However hungry or thirsty he