166 The Unity of the Organism 



environment of the organism. This is important in view 

 of the large use made of electricity in experimental work 

 on nerve physiology. To what extent may accepted gen- 

 eralizations concerning various reactions be influenced by 

 the inadequacy of the stimuli applied? 



We glance next at the physiological character of the con- 

 ducting member of the arc. It is hardly necessary to say 

 that typically this part is found in the nerve cords every- 

 where present in the body. The most general truth of sig- 

 nificance for us is the fact that having regard to conduction 

 through the arc as a whole, the process is different in many 

 important respects from conduction in the nerve cord alone. 

 Conduction in the arc as an integrated whole is quite dif- 

 ferent from conduction in the part of the arc specially 

 devoted to that office. To illustrate by a quotation : "An- 

 other remarkable difference between reflex-arc conduction 

 and nerve-trunk conduction is the irreversibility of direc- 

 tion in the former and the reversibility of the latter." 

 This is only another way of expressing the well-established 

 fact that nerve impulses are incapable, in the higher organ- 

 isms especially, of running in more than one direction. 

 Sensory excitations can go only centripetally and motor ex- 

 citations only peripherally. As Sherrington points out, this 

 is part of the "law of forward direction" of the neural system. 

 It is fully established that nerve impulses may run in both 

 directions when a stimulus is applied at any point or a 

 nerve trunk, whether the trunk be motor or sensory. Taken 

 by itself this fact might encourage a somewhat careless ob- 

 server to think of the nervous system as more or less hit-or- 

 miss in its structure and action, it being able to work one 

 way as well as another, the final result being determined by 

 where the stimuli happen to be applied. As a matter of 

 fact, though, when we come to consider the real unit of 

 neural organization, the reflex-arc, instead of any of the 

 constituents of that arc taken separately, all suggestion of 



