THE CENTRAL NERVOUS SYSTEM. 85 



on its back. When the animal is placed in water, it swims to the 

 margin and crawls out ; if it is placed on an inclined plane, it crawls to 

 the top and balances itself there. If, however, it is not stimulated in 

 any way, it will remain in the same attitude until it dies. The com- 

 plicated reactions known as volitional impulses are wanting, and the 

 frog shows no spontaneous movements. 



Similar phenomena may be observed in a pigeon from which the 

 cerebral hemispheres have been removed. There is the same mainten- 

 ance of posture unless the animal is disturbed ; and the power of 

 equilibration is not affected. The pigeon flies in a normal manner if 

 it is thrown in the air, but it soon alights and resumes its resting 

 attitude. It pecks at the ground if it is hungry, but does not feed 

 itself. 



The removal of the cerebral hemispheres in mammals is usually 

 followed by a fatal result, but Goltz succeeded in performing the 

 operation in a dog by carrying it out in successive stages ; and he 

 afterwards kept the animal alive for a year and a half. Temporary 

 paralysis followed, but was recovered from, and thereafter, in marked 

 contrast with the frog and pigeon under similar circumstances, the dog 

 showed a tendency to be in continual restless movement. It even 

 learned to feed itself when food was placed near its nose. It responded 

 to stimuli, if painful, by growling or barking and turning its head 

 towards the stimulated spot, though it showed no sign of recognition 

 of the persons who fed it, and gave no indication of fear when 

 threatened or of pleasure when caressed. 



The absence of the cerebral hemispheres, therefore, in the frog, 

 pigeon, and dog is associated with a condition in which the animal 

 responds to stimuli in a more direct and simple fashion than is the 

 case when the brain is intacjb. In the normal animal, the effect of a 

 stimulus is modified by impulses arising out of the memory of previous 

 experiences. When the cerebral hemispheres have been removed, the 

 memory records are absent, and the response to the stimulus is 

 simplified : in other words, there is an absence of intelligence, of 

 volition, and of emotion. But the machinery for the carrying out of 

 muscular movements in a co-ordinate manner still remains, and can be 

 set in action by a suitable stimulus. 



The function of the cerebral hemispheres is therefore associative, 

 combining the effects of immediate with those of past stimuli, and 

 giving out efferent impulses based on such combinations. The grey 

 matter of the cortex is not only excited by stimuli, but the stimuli 

 produce a permanent record in its cells, known as memory, which 

 exercises an important influence on all subsequent actions. 



