172 The Unity of the Organism 



source. "While the receptive neurone forms a private path 

 exclusively serving impulses of one source only, the final 

 or efferent neurone is, so to say, a public path, common 

 to impulses arising at any of many sources of reception." ^ 



An example which illustrates the general principle of the 

 common path, and several phenomena incidental to this, 

 and one which has been much investigated by Sherrington, 

 is that of the scratching reflex in dogs. It has long been 

 known that in several land vertebrates which have the 

 habit of scratching the side and back of the forward part 

 of the body with the hind foot, the scratching movement 

 may be elicited by appropriate stimuli applied to the area 

 reached by the foot as a pure reflex ; that is, in the absence 

 of any chance for impulses from the brain to reach the 

 parts involved in the activities. Since the scratching move- 

 ment consists in bringing the hind leg forward and upward, 

 and for scratching, a rhythmic movement of the foot, the 

 muscles, both flexor and extensor, of the thigh, leg and 

 foot must be involved. And since the reflex can be induced 

 by a stimulus applied at any point within the large recep- 

 tive field (i. e., nearly the whole side and back of the body), 

 impulses started from various parts of the field must pass 

 through one and the same neurone in the muscles concerned. 



And here comes in a fact showing another aspect of the 

 integration of reflexes in this case. A stimulus at a given 

 point in the field too weak by itself to elicit the reflex may 

 bring it on when acting in combination with weak stimuli 

 at other points in the field. Sherrington calls reflexes 

 which act together in this way allied reflexes. 



Still another kind of combination of reflexes involving 

 the common path principle, even more significant than allied 

 reflexes, are what are known as proprio-ceptive reflexes. 

 The kernel of this class of reactions is the existence of re- 

 ceptors in the deep tissues of the body, that is, not belonging 

 to the surface and hence not subject to stimuli from the 



