QOO THE CENTRAL NERVOUS SYSTEM 



afferent neurons, running from the receptive surfaces to the centres, 

 constitute each for its own receptive point a ' private ' path which 

 can only be used by impulses arising at that point, and not by 

 impulses arising at any other point. Through its central connec- 

 tions an afferent neuron from a single point may be put into com- 

 munication with numerous efferent neurons, and thus with numerous 

 and distant effector organs (muscles or glands). Conversely, the 

 efferent portion of a single reflex arc can convey reflex excitations 

 originating in numerous and distant receptive fields. It is the sole 

 path which all efferent impulses let them originate where they may 

 must use to reach the end-organ in question. It is therefore not a 

 private but a public path, and may be termed in this relation the 

 final common path (Sherrington). 



The existence of the common path is of great importance in under- 

 standing the manner in which reflexes are compounded together, a 

 problem absolutely fundamental in nervous co-ordination. One 

 consequence of the existence of a common path is that when, among 

 the receptors which may use it, two are simultaneously stimulated 

 which, when separately excited, produce opposite effects upon the 

 sffector organ, only one of the effects is produced. In other words, 

 impulses which produce the two opposed effects can be successively, 

 but cannot be simultaneously, sent along the common path. Thus, 

 ' excitation of the central end of the afferent root of the eighth or 

 seventh cervical nerve of the monkey evokes reflexly in the same 

 individual animal sometimes flexion at the elbow, sometimes ex- 

 tension. If the excitation be preceded by excitation of the first 

 thoracic root, the result is usually extension; if by excitation of the 

 sixth cervical root, it is usually flexion. Yet though the same root 

 may thus be made to evoke reflex contraction of the flexors or of the 

 extensors, it does not evoke contraction in both flexors and extensors 

 in the same reflex response. . . . The flexor-reflex, when it occurs, 

 seems, therefore, to exclude the extensor- re flex, and vice versa. 

 Either the one or the other results, but not the two together ' 

 (Sherrington). It is obvious that this is an advantageous arrange- 

 ment. An algebraical summation of the opposed effects by the 

 common path would result in a useless action which was neither 

 effective flexion nor effective extension, a compromise and not a 

 co-ordination. The conditions which determine which of two or 

 more competing reflexes shall obtain possession of the final comi 

 path are considered on p. 902. 



Role of the Receptor in Reflex Action. The role of the 

 the reflex arc is above all to sift out from the various kinds of im- 

 pressions impinging upon the receiving surface the particular kind 

 to which the appropriate response is the reflex action in question. 

 As will be pointed out in greater detail in the study of the special 

 senses, each kind of afferent end-organ has become adapted to a 



