Integrative Systems 



193 



serving to support or protect the sense-cells or facilitating access of 

 stimuli to them. These accessory structures will be described in the 

 chapters dealing with the animals in which they occur. 



CUTANEOUS SENSE-ORGANS 



Receptors existing in large numbers and widely distributed in the 

 skin are designated as "general cutaneous." 



At least four senses, pressure or 

 touch, warmth, cold, and pain, are 

 mediated by cutaneous organs. Corre- 

 sponding with these sensations are sev- 

 eral sorts of nerve-terminations. The 

 simplest and most common consists 

 merely of the finely branched terminal 

 fibrils of the afferent nerve itself, free 

 nerve-termination (Fig. 163). In 

 others the end of the nerve is related 

 to a specialized cell or a group of cells. 



Free nerve-terminations in the 

 dermis may become encapsuled by con- 

 centric layers of connective-tissue cells, 

 as in the so-called "corpuscles" of 

 Pacini, Krause, and Golgi-Mazzoni (Figs. 164, 165). Some nerve- 

 terminations are associated with tactile cells which are usually in the 

 dermis. In some instances, a single lenticular tactile cell rests upon a 

 cup-shaped termination of a sensory neurite, or the nerve may branch 

 among a cluster of such cells (Fig. 164, D-D 2 ). Such a cluster of 

 tactile cells, connected with the dendrites of a sensory nerve, may 

 become encapsuled by connective tissue to form a "Meissner's 

 corpuscle" (Figs. 164, D 3 ; 166). 



Free nerve-terminations may lie in the epidermis or in the dermis. 

 They are found in all classes of vertebrates and are believed to be the 

 sensory mechanism of painful sensations arising in the skin. The sense 

 of touch or pressure probably depends chiefly upon the tactile cells 

 or corpuscles in the dermis. 



In birds and reptiles, nerve-endings are connected with tactile 

 "cells of Merkel" but without a connective-tissue capsule. These tac- 

 tile cells are sometimes solitary, sometimes clustered. The Grandry's 

 corpuscles of birds are encapsuled, the nerve-termination lying be- 

 tween two tactile cells. 



In the relatively small corpuscles of Krause and in the large one 

 of Pacini, both found in mammals, the nerve-termination is club- 



Fig. 163. Free nerve-termi- 

 nations in the skin of Sala- 

 mandra. (After Retzius. Cour- 

 tesy, Kingsley: "Comparative 

 Anatomy of Vertebrates," Phila- 

 delphia, The Blakiston Com- 

 pany.) 



