844 THE CENTRAL NERVOUS SYSTEM 



cles do not undergo any more atrophy than can be accounted for by 

 disuse. The extremity does not suffer from any of the nutritional dis- 

 turbances which we saw supervene upon destruction of the motor cen- 

 ter in the cord; and likewise local reflex actions elicited by stimulation 

 of the local receptors are perfectly normal. A pinprick, for instance, 

 causes the usual flexion reflex. 



After some weeks the limb begins to recover and can be used in 

 volitional movement. Recovery rapidly progresses until, in the case of 

 the higher apes, it becomes almost complete in a little over four months. 

 It occurs earlier in the lower animals. When a center is destroyed on 

 the cerebral cortex in the case of man, only partial recovery takes place. 

 So that in general we may say that the higher the animal in the animal 

 scale, the less complete will be recovery from the paralysis produced by 

 cerebral ablation. 



Regarding the nature of the recovery, several possibilities exist: either 

 the nerve centers become regenerated in the destroyed area, or the cor- 

 responding area of the opposite hemisphere or some other part of the same 

 hemisphere or the basal ganglia assume the function. Evidence has been 

 furnished by Sherrington and Graham Brown 12 tending to show that 

 the last of these is the most likely cause for the recovery. Thus, it was 

 found, in working on the arm centers on the brain of the chimpanzee, 

 that after complete recovery of the paralysis produced by removal of 

 the center on one side, stimulation of the area that had been removed 

 caused no movements, indicating that no regeneration had occurred, and 

 that removal of the corresponding center of the opposite hemisphere, 

 although followed by paralysis of the arm to which it corresponded, still 

 did not cause any paralysis of the limb which had recovered from the 

 previous operation. To see whether some other part of the gray cortex 

 might have assumed the lost function, the postcentral convolution was 

 removed two months after the removal of the arm centers. Although 

 a temporary weakness of both arms resulted, the voluntary movements 

 were soon as good as before. These results are of course exactly what 

 we should expect from the experiment on the dog, already described 

 in which the cerebral cortex had been entirely removed, and the conclusion 

 that we must draw is that the basal ganglia assume the function of the 

 lost cerebral cortex. 



STIMULATION OF THE MOTOR CENTERS 



To investigate the effects of stimulation, it is found that the stimulus is 

 best applied by the electrical method, one pointed electrode, called the stim- 

 ulating, being applied to the area under investigation, and the other, 



