6io THE POPULAR SCIENCE MONTHLY. 



omy of the brain, thus studied, gives a clear indication that the differ- 

 ent 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 sen- 

 sory 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 con- 

 scious perception of a sensation. The conscious perception does not 

 occur in the organ irritated, nor in the nerve which carries the irrita- 

 tion. 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 w^here 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 ii'ritation 

 produced the sensation. 



Being perceived, the sensation is in some way registered and pre- 

 served, 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 mem- 

 ory must be independent of each other, and one may be lost while oth- 

 ers 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 permanent- 

 ly 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 poste- 

 rior 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 w^ay 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 



