FUNCTIONS OF THE CEREBRAL HEMISPHERES 499 



voluntary movement corresponding in distribution to the seat of the 

 lesion and proportionate in its severity to the extent of the lesion. 

 On the other hand, equally extensive lesions outside the ascending 

 frontal convolution have been shown to have no effect on volun- 

 taiy movements. The loss of movement is chiefly confined to those 

 which we regard as volitional. Although, for instance, the arm may 

 be paralysed, it can be still raised in association with a movement 

 involving the other arm. A certain degree of recovery from the 

 immediate effects of the lesion may be observed, but the recovery is 

 never complete. 



The difference in the reaction of various animals to Jesions of the 

 motor cortex is connected with the gradual shifting of functions 

 from the sphere of fatal necessary reactions to the sphere of educat- 

 able adaptations (i.e. from the lower centres to the cerebral cortex), 

 which is a characteristic of the evolution of the higher type of nervous 

 system, and is a concomitant of the increased adaptability which 

 distinguishes man from all the lower animals. In the animal without 

 hemispheres the motor mechanisms for all the movements of the 

 body are present and can be set into action from any point on the 

 sensory surface of the body. The first effect of adding the cerebral 

 hemispheres to this mechanism is to increase the range of reactions, 

 to modify them or to inhibit them, by diverting the stream of nervous 

 impulses into channels which have to a large extent been laid down 

 in the cortex by the past experience of the individual. In the frog 

 and bird we notice an automaticity and a ' conscious ' adaptation of 

 movements to purpose, although the hemispheres have no direct con- 

 nection with the motor centres of the cord, and present no areas 

 which we can designate as motor. In the dog, although a portion of 

 the brain is in direct connection with the spinal motor centres, and 

 can therefore initiate movements without making use of the mid- 

 brain motor machinery, these movements play only a small part 

 in the motor life of the animal, and the removal of the corresponding 

 centres takes away but little of the conscious functions of the animal. 

 In man the enormous power of acquisition of new movements is 

 rendered possible by the shifting of one motor function after another 

 to the sphere of influence of the cerebral hemispheres. Almost every 

 act of life in man has become one involving co-operation of the cerebral 

 cortex. For many years after birth man is helpless and far inferior, 

 as a reactive organism, to animals much lower in the scale. Even 

 the lower motor functions, such as those of locomotion or defence, 

 have to be painfully learnt, and this learning implies the laying 

 down of paths (Bahnung) in the cortex. On this account the sub- 

 cerebral centres in man are no longer complete. Acting in every 

 instance of life as a subordinate or adjunct to the cerebral hemispheres, 



