FUNCTIONS OF THE CEREBRAL HEMISPHERES 441 



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 lesions of the motor 

 cortex is connected with the gradual shifting of functions from the sphere 

 of fatal necessary reactions to the sphere of educatable adaptations (i.e. from 

 the lower centres to the cerebral cortex), which is a characteristic of the evolu- 

 tion of the higher type of nervous system, and is a concomitant of the in- 

 creased 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 hemi- 

 spheres 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 auto- 

 maticity and a ' conscious ' adaptation of movements to purpose, although 

 the hemispheres have no direct connection 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 enor- 

 mous 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 loco- 

 motion 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- 

 cortical centres in man are no longer complete. Acting in every instance 

 of life as a subordinate or adjunct to the cerebral hemispheres, they are unable 

 to carry out even the simpler motor reactions of the body after removal 

 of those portions of the hemispheres especially engaged in the control of 

 voluntary movement. The motor defect therefore which is brought about 

 in man, as a result of destruction of one or more of the motor centres, is to a 

 large extent permanent. 



If the lesion in man be strictly limited to the motor areas in the ascending 

 frontal convolution, it is impossible to detect any loss of sensation in the 

 affected parts of the body. On the other hand, some loss of sensation is 

 often found where the paralysis is widespread and occasioned by extensive 

 lesion in the neighbourhood of the Rolandic area. Moreover, even in localised 

 lesions in man, an epileptic fit may be preceded by a sensory aura in the part 



