THE FUNCTIONAL UNIT OF THE NERVOUS SYSTEM 575 



their nature. If an ameba retracts its pseudopodia or if a rhizopod 

 sends out its protoplasmic filaments into the surrounding medium, 

 stimulations of some sort must have taken place directly preceding 

 these responses. But as these excitations have resulted in living 

 substance which is free from nervous elements, the reactions, although 

 just as complex as many of those exhibited by the higher animals, can 

 only be said to be reflex-like in their character. 



The other group of organisms, embracing those possessing nervous 

 elements, shows a gradually increasing complexity in the arrangement 

 of its nervous units and also a steadily increasing complexity in its 

 reactions. The simplest of these are designated as reflexes and the 

 most complex, as associated actions or voluntary reactions. The divid- 

 ing line between these processes lies in volition. Thus, we may use 

 the term reaction in a very general way as designating any response 

 to a stimulus, but, more correctly speaking, it should be restricted 

 to that response which is accomplished with the aid of the will. A 

 reaction, therefore, is a volitional action, while a reflex is an action 

 which is not influenced by volition. To summarize, the different 

 actions shown by animals may be divided into reflex-like actions, 

 reflexes and complex reactions. The first of these are had solely with 

 the aid of ordinary protoplasm, while the last two necessitate the pres- 

 ence of that differentiated type of living substance which we call 

 neuroplasm. Furthermore, as long as an action of the latter kind is 

 not influenced by the will, it remains a reflex, but becomes a complex 

 reaction immediately upon the entrance of volition. 



The Reflex Circuit. It need scarcely be emphasized that the pres- 

 ent discussion must be restricted very largely to the analysis of the 

 nervous activities of the higher forms and hence, reflex-like actions 

 must be left for later consideration. The phenomena of life have been 

 divided into spontaneous manifestations and manifestations of stimu- 

 lation. Strictly speaking, however, this classification is incorrect, 

 because life consists in a reaction of living substance to outside in- 

 fluences. Hence, stimulations are always present and a state of abso- 

 lute spontaneity cannot arise. Stimuli are constantly brought to 

 bear upon organisms and it is their destiny to react toward them in 

 accordance with their structural and functional equipment. Moreover, 

 if we define a stimulus as any extraordinary alteration in the conditions 

 which nature has imposed upon us, we must immediately be struck 

 by the enormous diversity of influences to which we may be subjected. 

 Animals, very naturally, react toward these changes in harmony 

 with the development of their nervous system. The lower forms being 

 constructed along much simpler lines, are essentially reflex animals, 

 for the reason that their psychic activities are lacking and their actions 

 cannot, therefore, be dominated by the will. The higher animals, 

 on the other hand, are reaction-animals, because their psychic life 

 absolutely controls their simple reflex functions. 



A reflex is a response to a stimulus executed without the interven- 



