THE RECEPTORS 



855 



Just as a division of labor has been necessary between the receptor 

 and afferent fiber in order that the former may become as sensitive as 

 possible to weak stimuli, so we find a specialization among the receptors, 

 each being adapted to respond to some particular kind of physical or 

 chemical change in its surroundings. Thus we may classify receptors 

 with respect to the stimuli to which they respond most readily. We 

 may speak of chemo-receptors which respond to chemical changes 

 (taste, smell), tango-receptors which respond to pressure (touch), photo- 

 receptors which respond to light (sight), phono-receptors which respond 

 to sound (hearing), and caloro-receptors which respond to temperature 

 changes (heat and cold). This does not mean, however, that these sense 

 organs respond only to the type of stimulus to which they are especially 



Fig. 212. Evolution of the sense organs. I. Receptor of coelenterate. II. Type of receptor 

 found in many invertebrates and in gustatory epithelium of vertebrates. III. Simple afferent 

 neuron of vertebrate with free nerve terminations in skin. IV. Specialized sensory mechanism of 

 vertebrate, typified by the organ 'of the sense of taste. It consists of specialized receptor cells in 

 the integument, about which terminates the axon of an afferent neuron. (After Parker.) 



attuned. Pressure upon the eyeball may stimulate the retina, an elec- 

 tric current may give rise to the sensation of taste, a temperature of 45 

 C. will excite the sense organs not only of heat but of cold and pain. But 

 in the latter case, for example, the heat receptors alone are responsive 

 to temperatures between 45 C. and the temperature of the body. The 

 characteristic of the specialized receptor is that its threshold or lower 

 limit of stimulation, is much lower for one type of stimulus than for any 

 other. 



The Quality of Sensation and Its Local Sign 



Whatever the form of stimulus which excites a given receptor, the 

 quality of the sensation is always the same. A temperature of 45 C. 



