96 VETERINARY PHYSIOLOGY 



If the special part of the nervous system with which any- 

 set of receptors is connected be destro^'ed or become discon- 

 nected from the receptors, stimulation will produce no effect. 

 The condition is that of a telephone with the transmitter 

 intact but with the receiver out of action. 



The study of the physiology of the peripheral receptors 

 must therefore be taken up along with that of those parts 

 of the central nervous system with which they are connected. 



These, when thrown into action through the receptors, may 

 (1) produce certain inevitable reactions, as has been shown 

 by the study of reflex action (p. 82), and these may or may 

 not (2) be accompanied by changes in the consciousness 

 known as sensations, which cannot be investigated in lower 

 animals. 



Receptor Arrangements. 



The Receptors may conveniently be grouped in three 

 classes. 



1. Those connected with the viscera and stimulated by 

 the activitv of these organs — the Intero-ceptive Receptors. 



2. Those connected with the surface and stimulated 

 by changes in the surroundings — the Extero-ceptive Receptors. 



Some are acted upon by changes at the surface of the 

 body, such as the contact of gross matter, others by changes 

 originating at a distance, e.g. light waves. The former may 

 be called non-distance receptors — the latter distance receptors. 



3. A set of receptors called into play by movements of the 

 body, i.e. by the action of the body itself, and hence called 

 Proprioceptive Receptors. 



We have already seen that each special effect is brought 

 about by the development of special receptors, each 

 responding chiefly to one kind of external change (p. 52). 



It is not the case that each reacts to one kind of change 

 only, but rather that it reacts specially to one kind, that it 

 is tuned to one kind and responds much less readily to all 

 others. As Sherrington puts it, it has a " low threshold " 

 for one kind of stimulus, a " high threshold " for all others. 

 Thus, the eye is generally stimulated by the ethereal vibra- 



