66 INTRODUCTION TO NEUROLOGY 



reflex path; but the stimulus is rarely simple and the nervous 

 discharge irradiates more or less widely, so that the activity is 

 by no means limited to the part which gives the act its reflex 

 pattern. Moreover, neither the stimulus complex nor the char- 

 acter of the irradiation will be repeated exactly in any higher 

 animal, so that the precise nature of the response cannot in any 

 case be infallibly predicted except under experimental conditions 

 (and not always then). 



Our picture of the reflex act in a higher animal will, then, 

 include a view of the whole nervous system in a state of neural 

 tension. The stimulus disturbs the equilibrium at a definite 

 point (the receptor), and the wave of nervous discharge thus set 

 up irradiates through the complex lines determined by the neural 

 connections of the receptor. If the stimulus is weak and the 

 reflex path is simple and well insulated, a simple response may 

 follow immediately. Under other conditions the nervous dis- 

 charge may be inhibited before it reaches any effector, or it may 

 irradiate widely, producing a very complex reflex pattern. In 

 the former case the neural equilibrium will be only locally 

 disturbed; in the latter case almost the whole nervous system 

 may participate in the reaction, a part focal and sharply defined 

 and the rest marginal, diffuse, and exercising more or less of 

 inhibitory or reinforcing control on the final reaction. 



The studies of Herrick and Coghill have shown that in the 

 development of the nervous system of Amphibia the first reflex 

 circuits to come to maturity are made up of rather complex 

 chains of neurons so arranged as to permit of only one type of 

 response, viz., a total reaction (the swimming movement), from 

 all possible forms of stimulation, and that in successive later 

 stages this generalized type is gradually replaced by a series of 

 special reflexes involving more diversified movements. Parallel 

 with this process the higher correlation centers are developed 

 for the integration of the several special reflexes into complex 

 action systems. The simple reflex arc, as illustrated in Fig. 

 1 (p. 25), which is adapted for the execution of a single movement 

 in response to a particular stimulus, is the final stage in this 

 developmental process, whose initial stages are much more 

 complex and diffuse arrangements of neurons adapted for total 

 reactions of a more general sort. 



