186 THE HUMAN BODY 



memory are supreme we call strong-willed. The weak-willed per- 

 son yields to temptations which are powerless to move the one 

 whose will power is great. In this definition of will power we have 

 set associative memory against immediate stimulation, and usually 

 the conflict is between these. Sometimes, however, the struggle 

 comes between different immediate stimuli. In the weak-willed 

 man the more insistent ones are likely to control, rather than the 

 more important. If he is beseeched by various friends to accom- 

 pany them different ways the most vociferous is usually the one to 

 carry him off. The strong-willed man, on the other hand, makes his 

 decision on other grounds. On the other hand, various associative 

 memories may be in conflict, and here again, whether obedience 

 will be to the most clamorous or the most important depends on 

 the strength of the will. 



Cerebral Control of Spinal and Cerebellar Reflexes. The 

 exercise of the inhibitory function of associative memory as just 

 described involves an ability on the part of the cerebrum to modify 

 the reflexes of the lower parts of the nervous system. There is 

 abundant evidence that such ability actually exists. In the case 

 of spinal reflexes it may be supposed to act through the discharge of 

 impulses from the motor area which in some manner increase 

 synaptic resistances in the course of the reflexes sufficiently to 

 block them. A feature of the inhibition of spinal reflexes which 

 points to this as the means of bringing it about is that in the case of 

 sharp sensory stimulation the inhibition must be established before 

 the stimulus is received. If one unexpectedly touches a hot object 

 he automatically and inevitably jerks his hand away, but if he 

 knew the object was hot, and nevertheless found it necessary to 

 grasp it he could, through an act of volition, block his reflex path so 

 effectively that the tendency to draw the hand away would be 

 completely overcome. 



Cerebellar reflexes are also subject to cerebral control. There 

 is evidence that part of the pyramidal tract from the motor area 

 terminates in the brain stem in relationship with paths leading into 

 and out of the cerebellum. Apparently thus voluntary control of 

 locomotion is exercised. That we have such voluntary control 

 is evident. We can start, stop, or modify our locomotor acts at 

 will, although as we have previously seen, the performance of the 

 reflexes as distinct from their guidance, is automatic. 



