THE NERVOUS SYSTEM 63 



the stimulus is progressively lowered by each succeed- 

 ing application, and the facility of energy discharge 

 is correspondingly increased until a period of maximum 

 discharge is reached. Another example of summation 

 is the increasing sensitiveness to pain felt by a patient, 

 who for a long time has had to submit to frequent pain- 

 ful wound dressings. 



In a larger sense, the behavior of the individual 

 illustrates summation. The facility with which energy 

 is discharged in response to any stimulus is the result 

 of the evolution of the responding mechanism through 

 natural selection (phylogeny) ; and of the individual's 

 own past life (ontogeny). Moral as well as physical 

 efficiency may justly be regarded as the result of 

 summation. In the training of athletes, where su- 

 perior strength of certain sets of muscles is desired, ef- 

 ficiency is the result of the repetition of exercises which 

 involve muscular action activation of certain action 

 patterns at such intervals that the upbuilding effect 

 of one action is not lost before a new exercise is given. 

 Thus a gradual ascent to the maximum efficiency is 

 realized. The principle of summation plays a similar 

 important role in the production of certain pathologic 

 phenomena, a progressively lowered threshold being 

 largely responsible, for example, for such conditions as 

 neurasthenia and mania. 



An understanding of the mode of action of adequate 

 stimulus, final common path, threshold and summation, 

 upon which are constructed the action patterns which 

 constitute man's motor adaptation to environment, 

 assists in making more clear much that seems complex 

 and confusing in human action. 



