FUNCTIONS OF THE CEREBRAL HEMISPHERES 501 



afferent fibres from the posterior roots by which similar movements 

 may be evoked. 



On the other hand, the activity of the pyramidal cells of the cortex, 

 like those of the motor-cells of the spinal cord, is determined by the 

 arrival at them of afferent impressions. In the absence of these 

 afferent impressions no spontaneous discharge of motor impulses 

 takes place. Thus in the spinal frog we have seen that complete 

 inactivity is brought about by section of all the posterior roots. In 

 the same way paralysis of the arm is induced by section of all its 

 posterior roots, although it can be shown that the motor cortex is still 

 excitable, and that the application of an induced current to the 

 motor centres of the arm evokes a movement as easily as in the normal 

 animal. The motor-cells in the cortical motor centres are normally 

 played upon and aroused by impressions arriving at them from all 

 other parts of the brain and nervous system, and determined originally 

 by impressions falling on the surface of the body. 



THE LOCALISATION OF SENSORY FUNCTIONS IN THE CORTEX 



It was pointed out by Ferrier that movements might be obtained 

 on electrical excitation of regions of the cortex cerebri other than 

 those we have described as motor. Thus excitation of the superior 

 temporal convolution on the right side causes the animal . to 

 turn its head and eyes to the left and to prick up its ears. In the 

 same way stimulation of the right occipital lobe causes movement 

 of both eyes and head to the left side. These portions of the brain 

 cannot be regarded as having a direct relationship to the motor 

 mechanisms involved in the above movements, since their ablation 

 leads to no defect of movement, but does, in many cases, lead to 

 defect of sensation. Thus excision of the whole of the occipital lobe 

 in the monkey, though leaving the eye movements intact, causes a 

 loss of power to discern objects lying to the left of the middle line. 

 The obvious explanation therefore of the movements, obtained on 

 excitation of this portion of the cortex, is that they are due to the 

 revival or arousing of sensory impressions, that these portions of the 

 cortex represent the cortical receiving stations for the impulses from 

 definite sense-organs, and that the movements obtained are simply 

 those which are normally associated with a corresponding sensory 

 excitation. 



This conclusion is borne out by the fact that it requires a greater 

 strength of stimulus to excite movement on stimulation of the sensory 

 areas than is necessary if the stimulus be applied to the Rolandic 

 area. Moreover Schafer has shown that the latent period which 

 intervenes between the stimulus and the resulting movement is 

 considerably longer when the stimulus is applied to the sensory centre 



