798 A MANUAL OF PHYSIOLOGY 



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 connections 

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

 tion 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 

 understanding 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 effector 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 indi- 

 vidual 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 con- 

 traction 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-reflex, and vice versa. Either the one or 

 the other results, but not the two together ' (Sherrington). It 

 is obvious that this is an advantageous arrangement. An alge- 

 braical 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 role of the receptor in the reflex arc is above all to sift 

 out from the various kinds of impressions 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 special, or, as 

 it is termed, an ' adequate ' stimulus, so that it is easily affected 

 by this, and with difficulty or not at all by other modes of stimula- 

 tion. Thus, light is the adequate stimulus of the end-organ of 



