FUNCTIONS OF THE CEREBRAL HEMISPHERES 451 



mechanisms, viz. the pyramidal tracts, should determine increased tonus and 

 rigidity of the limbs the so-called ' spastic ' condition observed in cere- 

 bral paralyses. 



Two separable systems of motor innervation appear thus to control two 

 sets of musculature. One system exhibits the transient phases of heightened 

 reaction which constitute reflex movements ; the other maintains that steady 

 tonic response which supplies the muscular tension necessary to attitude. 

 Hughlings Jackson long ago called attention to this contrast between the 

 two systems. He pointed out that while the cerebrum innervates the muscles 

 in the order of their action from the most voluntary movements (the limbs) 

 to the most automatic (trunk), the cerebellum, or, as we should say now, 

 the whole proprioceptive system, innervates them in the opposite order. 

 The cerebellum therefore he regarded as the centre for continuous move- 

 ments and the cerebrum for changing movements. The increased tone of the 

 paralysed muscles, observable after hemiplegia, 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 

 ex irpationof 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 its past history, and this modifying influence on 

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

 scious motor act we may say therefore that the brain functions 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 te 



