"Responses of Higher Animals: 

 The Receptors 



THE PRECISE and rapid responses of com- 

 plex animals would not be possible in the 

 absence of a highly developed nervous sys- 

 tem; the nerve cells of the body are con- 

 stantly engaged in relaying excitations from 

 the sense organs and other receptors to the 

 muscles and other effectors of the body. First 

 to be considered, therefore, are the sense or- 

 gans and other receptors, which initiate most 

 excitations. 



THE SENSE ORGANS AND OTHER 

 RECEPTORS 



In man, the skin alone houses five distinct 

 kinds of receptors — for touch, pressure, pain, 

 warmth, and coldness; and in the head, there 

 are the special sense organs of sight, hearing, 

 taste, smell, and balance. Moreover, the mus- 

 cles throughout the body are equipped with 

 receptors of pressure, tension, and pain, and 

 there are still other kinds of receptors, which 

 will be mentioned later. 



Cutaneous Receptors. Under the micro- 



scope a section of skin displays well-defined 

 receptors for touch, pressure, coldness, and 

 warmth (Fig. 23-1A.B), as well as many free 

 nerve endings, which are susceptible to 

 direct stimulation by various pain-inflicting 

 agencies (Fig. 23-2). These cutaneous re- 

 ceptors can also be located by testing the 

 sensitivity of the skin point by point, on the 

 arm for example. A stiff bristle is used to 

 demonstrate the tactile receptors, and a 

 sharp needle will show the pain receptors; 

 but a blunt metal stylus is required for the 

 pressure and temperature receptors. The 

 stylus is kept at body temperature in demon- 

 strating the pressure corpuscles, but the stylus 

 must be chilled or warmed in finding the 

 points that are sensitive to cold and heat. 

 Thus it is possible to plot a given area of 

 skin, marking the exact localization of each 

 kind of receptor; and such a plot shows a 

 separate locus for each different receptor 

 unit. 



Reception of a stimulus involves the ex- 

 citation of specialized cells in the sense organ, 



419 



