712 COMPARATIVE PHYSIOLOGY OF THE BRAIN [CH. L. 



In the bird, removal of the hemispheres and basal ganglia pro- 

 duces the sleepy condition already described (p. 679); when the 

 animal, is made to fly its movements are directed by the sense of 

 sight, and it will select a perch to settle on in preference to the floor. 

 It will start at a noise ; it will not eat voluntarily ; it exhibits no 

 emotions such as fear, sexual feeling, or maternal instincts. 



In mammals, the difficulty of the operation has been overcome by 

 G-oltz in dogs by removing the cerebrum piecemeal. One dog treated 

 in this way lived in good health for eighteen months, when it was 

 killed in order that a thorough examination of the brain might 

 be made. It was then found that not only the hemispheres but the 

 main parts of the optic thalamus and corpus striatum had been 

 removed also. Though it could still carry out co-ordinated move- 

 ments, its reactions were entirely reflex, and emotions, feelings, or 

 the capacity to learn were entirely absent. 



If we now compare these effects, it is seen that the results of the 

 operation becomes progressively greater as we ascend the scale. The 

 higher the animal, the more fatal the effects, the immediate disturb- 

 ance more severe, the return of function slower, and the permanent 

 loss greater. The long life of Goltz's dog was doubtless due to the 

 fact that the removal was accomplished by several operations. 



The higher animal loses just those characters which distinguish 

 it from the lower ones. It is difficult to prophesy what would 

 happen if as extensive operations were carried out in a monkey or a 

 man. But so far as extirpation has been observed, the initial paralysis 

 (which is seen also in the dog) does not disappear so rapidly or so 

 completely. In man, the tendency to recover is least. 



This is anatomically explicable when we remember that the 

 anterior horn cells are influenced chiefly by two sets of impulses, 

 those which enter the cord by the posterior roots, and those which 

 come down from the cerebrum by the pyramidal tracts. In the lower 

 animals the pyramidal pathway is insignificant, and when it is inter- 

 rupted the disturbance is consequently slight. In animals below 

 the mammals it is absent, and going up the mammalian scale it 

 becomes more and more important as the following figures show : 



In the mouse the pyramidal fibres constitute J/14 per cent, of those in the cord. 

 guinea-pig 3'0 



rabbit 



cat 776 



man 11 '87 



We can therefore quite readily understand that in the apes and 

 in man, a damage to the cortex which causes degeneration of these 

 tracts will cut off many impulses to the anterior cornual cells, and 

 produce a greater or less degree of paralysis. 



We have already pointed out (p. 086) that the size of the cortical 

 areas does not vary with the mere mass of the muscles under control, 



