FUNCTIONS OF THE CEREBRAL HEMISPHERES 509 



plegia, he ascribed to unbalanced cerebellar influence. While there 

 is no doubt that the cerebellum must play, and does play, a 

 considerable part in the production of decerebrate rigidity and 

 of the spastic condition of hemiplegia. it is not the only element 

 involved ; nor is it essential, since decerebrate rigidity may 

 continue after extirpation of the cerebellum and an exaggerated 

 knee-jerk may result from section of the spinal cord in the lower 

 cervical region. 



HIGHER ASSOCIATIVE FUNCTIONS OF THE CORTEX 

 The simple and uncomplicated nature of the movements elicited 

 on cortical stimulation shows that we cannot regard these motor 

 centres as responsible for the whole, or even the greater part, of the 

 motor functions of the cortex. They are in fact simply the starting- 

 point for the motor impulses which run down the long pyramidal 

 tracts, but which result from the activities of the cerebral hemi- 

 spheres as a whole. In the lower mammals they do not even repre- 

 sent the only starting-point, as is shown by the almost perfect recovery 

 of volitional motor power in a dog deprived of its motor cortex. 

 The distinguishing feature of the response of an animal possessing 

 cerebral hemispheres is that it is not determined solely and exclu- 

 sively by the nature and position of the peripheral stimulation, but 

 involves elements connected with the past experiences of the animal, 

 and including therefore the results of previous stimulation of many 

 of the sense-organs, either directly, or indirectly as a result of reflex 

 movements. The animal's reactivity is determined by the past 

 history of the animal, and this modifying influence on the brain 

 must involve parts connected with all its sense-organs. In any 

 conscious motor act we may say therefore that the brain acts as a 

 whole, or nearly as a whole. 



In endeavouring to arrive at some idea of the neural processes 

 concerned in volitional movements, i.e. movements of the intact 

 animal, we are dealing with events which in ourselves come within 

 the sphere of consciousness, so that some assistance is derived by 

 appealing to our own mental experiences. Especially is this necessary 

 in the case of the sensations. It might be imagined that a simple 

 sensation would ensue as the result of local stimulation, say of the 

 visual centre on one side. Our knowledge of the properties of the 

 systems of neurons composing the central nervous system would 

 teach us that no excitatory process could remain confined to one 

 portion of the brain, but must diverge in many directions. It is 

 true that excision of the occipital lobes on one side causes blindness 

 to objects in the opposite half of the field of vision. This is, however, 

 merely a result of localisation of the end of visual fibres, and the 



