144 THE MECHANISM OF LIFE 



cortex. It is not tlie same thing in the mammals as it is in the 

 birds and reptiles, where it is a more complex organ than it is in 

 the amphibians, and it does not occur at all among the fishes. 

 This disparity of development must always be remembered when 

 we discuss the functions of the cerebral hemispheres so far as 

 these can be made out from observations of the effects of opera- 

 tive interference. Removal has little or no apparent effect in a 

 fish, where the " cortex " is non-nervous, but the result is very 

 obvious in the frog, and still more so in mammals such as 

 the rat, rabbit, and dog. In the monkeys and apes, and, of 

 course, in man, the operation is an impossible one, for these 

 animals do not survive it long enough to make clinical study 

 profitable. 



In lower vertebrates, such as the frog, the cortex is relatively 

 unimportant (judged from the anatomical standpoint) when it 

 is compared with the rest of the brain, and the functions that it 

 subserves are also unimportant. When it is removed the change 

 of behaviour is not a profound one, and it is possible that some 

 of the things that were formerly done by the cerebral hemispheres 

 are then done (or partially done) by the mid-brain. To some 

 extent this is also the case with the dog, where vicarious function- 

 ing on the part of the lower brain may be set up when the cortex 

 is removed. That such is the case is suggested by the observa- 

 tion that animals deprived of their cerebral hemispheres tend to 

 act more normally the longer they can be kept alive and in good 

 general health. Now, in the monkeys and anthropoid apes, the 

 development of the means of control over acting by the cortex 

 has been carried so far that vicarious functioning by the mid- 

 brain and thalami become impossible, and this is still more the 

 case with man. 



Reviewing the bare summary of the evidence that has been 

 made here we see, however, that with the removal of the cortex 

 cerebri spontaneous movements tend to disappear, while auto- 

 matic and mechanical activities persist. The animal so treated 

 reacts to stimuli in a blind, inevitable manner, so that its responses 

 can be predicted. There is physical determinism — at any rate, 

 much more of such than in the intact, cerebrate animal. Our 

 only criterion of volition, deliberation, and intelligence is this 

 presence of spontaneous behaviour, and therefore we are justified 

 in placing the immediate expressions of will and intelligence in 

 the activities of the cortical mechanisms. 



