Neural Integration 165 



reflex effects can be excited [by fluid] from the very organs 

 the cutting and wounding of which remains without effect." 2 

 The reflex mechanism involved is adjusted to respond only 

 to a stimulus of a special kind. In this it is comparable to 

 the optical mechanism referred to above. 



These illustrations show something of the general nature 

 of the reflex-arc as an integrated structure. But we want 

 to know something about the part contributed by the 

 different constituents of the arc to this nature. The spe- 

 cific office of sense organs viewed as the receptor members of 

 the arcs will first engage attention. "The main function of 

 the receptor is therefore to lower the threshold of excit- 

 ability of the arc for one kind of stimulus, and to heighten 

 it for all others." 3 This principle is so important that we 

 must allow no vagueness as to its meaning. It means that 

 while more than one sort of stimulus may put a particular 

 sense apparatus into operation, there is one and only one 

 kind, namely, that to which the sense organ is naturally 

 subject, that elicits the reflex in its normal or type expres- 

 sion. A sense organ may be looked upon as a group of 

 receptors attuned to a special stimulus, as contrasted with 

 that of the general stimuli to which an organism is always 

 subject by being always in contact with its environment. 



In illustration Sherrington instances the fact that the 

 threshold for the touch-sensation is lower for a mechanical 

 stimulus than for an electrical stimulus. Having regard to 

 the whole lot of reflex-arcs of the body, we may say that 

 the different sense organs constitute mechanisms of selective 

 excitability for the different stimuli, each organ being so 

 adapted to its natural stimulus that it responds to this 

 better than to any other. This reference to the sense or- 

 gans as adaptive makes it desirable to notice the fact that, 

 according to Sherrington, electricity is never an adequate 

 stimulus of any sense organ because it is not a natural 

 stimulus for any, since it does not enter into the natural 



