FUNCTIONS OF THE CEREBRAL HEMISPHERES i:>| 



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 hemispheres as a whole. In the lower mammals 

 they do not even represent 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 exclusively 

 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 stimula- 

 tion, 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 same effect can be brought about by division of 

 the right optic tract, or damage to the right half of both retinae. 



On the other hand, an appeal to our own experience shows that no 

 sensation can be regarded as simple, i.e. as following merely stimulation of 

 visual fibres or visual centres. Thus the sensation of a luminous point 

 has connected with it not only luminosity but also colour and intensity. 

 Moreover the apparent position of the luminous point comes into conscious- 

 ness at the same time as the consciousness of the luminosity itself, and this 

 location of the stimulation involves muscular impressions from the eyeballs 

 and an association between certain points on the retina and certain corre- 



