THE NERVOUS SYSTEM. 



455 



rones may be conveniently termed receptors, those structures affected by the 

 efferent peripheral neurones may be termed effectors (Sherrington). Receptors 

 include various "sensory" structures whose principal function appears to be 

 to limit to some particular kind of stimulus the changes affecting the afferent 

 nervous elements connected with them. Effectors include various structures 

 (muscles, glandular epithelia) whose activities are influenced by the nervous 

 system (Fig. 396). A primitive nervous mechanism, thus composed of (1) 

 afferent peripheral neurones which transmit the stimulus from a receptor to 

 (2) efferent peripheral neurones which iii turn transmit the stimulus to an 

 effector, is a simple, two-neurone reflex arc (Fig. 396). 



At the same time these neurones, as they increase in number, are obviously 

 brought into relation with each other with more economy of space by having 



Receptor 



Fig. 396.— A two-neurone reflex arc in a Vertebrate, gg. Ganglion, van Cehuchten 



common meeting places. This, together with the factor noted below, leads to 

 the concentration of an originally diffuse nervous system, spread out principally 

 in connection with the outer (ectodermal) surface, into a more centralized 

 (ganglionic) type of nervous system, which at the same time has in part re- 

 treated from the surface layer (ectoderm) from which it was originally derived 



(Fig- 397)- rc 



Furthermore, when we consider the great number of receptors and effectors 



in even simple forms, it is apparent that for effective coordination there must be 



a considerable degree of complexity of association between the afferent and 



efferent neurones. These associations may be to some extent accomplished by 



various branches of the afferent and efferent neurones coming directly into 



various relations with each other, but it is also evident that when a certain 



