420 - Multicellular Animals, Especially Man 



EPITHELIUM 



NERVE ENDING 



PAIN RECEPTORS 



Fig. 23-2 Free nerve ending in epithelium. The ter- 

 minal branches of the afferent nerve fiber end freely 

 among the epithelial cells. 



Fig. 23-1. Some types of mechano- 

 receptors of vertebrates. A, tactile cor- 

 puscle (touch receptor) from the skin. 

 B, Pacinian corpuscle (pressure recep- 

 tor), found in deeper layers of the skin 

 and in many internal organs. C, Golgi 

 organ (proprioceptor) on a tendon. D, 

 muscle spindle (proprioceptor) in skele- 

 tal muscle, n, afferent nerve fibers; c, 

 connective tissue capsules; t, tendon; 

 m, muscle fibers. 



as is shown in the case of a tactile corpuscle 

 (Fig. 23-1 A). Such receptor cells are usually 

 separate from the sensory nerve cells, which 

 conduct the excitations to the central parts 

 of the nervous system; but in some cases — 

 lor example, the pain receptors — the sensory 

 nerve cells act as conductors as well as re- 

 ceptors (Fig. 23-2). 



The receptors of touch and temperature 

 are restricted mainly to the superficial parts 

 of the body, especially in the skin, and on 

 the lips and cornea; but pressure and pain 

 receptors are also found in many deep-lving 

 parts. 



Receptors of Taste and Smell. Each ani- 

 mal discovers and recogni/es its food by 

 means of chemoreceptors, which are ex- 



