348 ELEMENTARY PHYSIOLOGY less. 



area ; they are found, for instance, in the alimentary canaL 

 Already we have said that a protopathic sensation may 

 give the impression of coming from a different place from 

 that at which it actually originates. This is especially 

 true of sensations wliich come from the viscera. Such 

 are often felt to be in the skin, indeed it is a re- 

 markable thing that there are particular situations on the 

 skin corresponding to various organs of the body so that 

 a pain which really has its origin in the stomach will be 

 felt always in one skin-area, whilst a pain from the 

 kidney will be felt in another. This phenomenon is 

 known as that of "referred pain." 



Let us turn to the structure of the skin to ascer- 

 tain if possible the nature of the sensory end organs 

 in it. 



Whatever part possesses this sense consists of a mem- 

 brane (integumentary or mucous) composed of a deep 

 layer made up of fibrous tissue containing a capillary 

 network, and of a superficial layer consisting of epithelial 

 or epidermic cells, among which are no vessels. 



Wherever the sense of touch is delicate, the deep layer 

 is not a mere flat expansion, but is raised up into multi- 

 tudes of small, close-set, conical elevations (see Fig. 57, 

 p. 191), which are called papillae. In the skin, the coat 

 of epithelial or epidermic cells does not follow the contour 

 of these papillae, but dips down between them and forms a 

 tolerably even coat over them. Thus, the points of the 

 papilliB are much nearer the surface than the general 

 plane of the deep layer whence these papillae proceed. 

 Loops of vessels enter the papillae, and sensory nerve- 

 fibres are distributed to them. In some cases the nerve- 

 fibre ends in a papilla in a definite organ, in what is called 

 a tactile corpuscle, or in a similar body called an 

 end-bulb. Each of these organs consists essentially of 

 an oval or rounded swelling formed by a modification and 

 enlargement of the delicate connective tissue ensheathing 



