THE NERVOUS SYSTEM 57 



all life processes result from the immersion of a sensi- 

 tive structure in an activating environment. Ade- 

 quate stimulus represents the environment ; conduc- 

 tion represents the mechanism of communication ; and 

 end effect represents life itself in all its manifold ex- 

 pression. 



The production of a simple adaptive motor act in 

 response to an external physical stimulus is a reflex. 

 The combined processes represented by adequate 

 stimulus, conduction and end effect are known as reflex 

 arcs. There are many such reflex arcs in the organism, 

 which can be easily differentiated into their component 

 parts reception, conduction and end effect. Such 

 reflexes are the contraction of the iris in response to 

 light, and the winking of the eyelid in response to 

 excitation of the conjunctiva. There are other reflex 

 arcs, however, which, according to our conception, are 

 built on the same plan, but whose component parts 

 it is far from easy to determine. It is not easy, for 

 instance, to define the specific, visible physical agents 

 in the external environment which are responsible for 

 the reflex actions manifested by a benevolent deed, by 

 an artistic production, or by an intellectual process, 

 even though we know, in a general way, that by 

 natural selection the structures which manifest these 

 responses have been developed as those best fitted to 

 survive. Equally difficult is it to identify the forces 

 which are responsible for the development of the intri- 

 cate and obscure processes of metabolism, of respira- 

 tion, of heart beat, of immunity or of reproduction. 



