OF THE SENSE OF TOUCH. 109 



Having studied in a general way the phenomenon of the 

 sensibility, as well as the organs which are its seat, we ought 

 now to examine more in detail each of the forms under which 

 this property is manifested, or in other words, enter on the 

 particular history of each of the senses with which nature 

 has endowed animals. 



OF THE SENSE OF TOUCH. 



206. All animals possess a tactile sensibility more or 

 less delicate, and it is especially by the intermedium of the 

 membrane with which the surface of their bodies is covered 

 that this faculty is exercised. To study it, it is, then, 

 above all necessary to examine what is the structure of the 

 skin. 



In man, the external surface of the body, and that of the 

 cavities hollowed out in the interior, but communicating 

 externally, such as the digestive canal, &c., are clothed with 

 a tegumentary membrane more or less thick, and quite 

 distinct from the parts which it covers. This membrane is 

 everywhere continuous with itself, and in reality it forms but 

 a continuous whole; but its properties are not everywhere 

 the same, and it is called by different names when it is 

 reflected internally to line the interior cavities, or when it is 

 extended over the outer surface of the body. The internal 

 portion of the general tegumentary membrane is called 

 mucous membrane, and the external portion skin. 



207. Structure of the Skin. Two principal layers 

 compose the skin : the dermis or true skin, and the epidermis 

 or scarf skin. 



The dermis forms the deeper and thicker layer of the in- 

 teguments. It is a strong, supple, elastic membrane, whitish, 

 and very resistant. A great number of fibres may be seen 

 in it, crossing each other in all directions. Beneath it is a 

 dense layer of cellular substance connecting it to the subja- 

 cent parts, and in this sometimes fleshy fibres are found. On 

 its surface may be seen, especially in the palms of the hands 

 and extremities of the fingers, the elevations, called papillae, 

 arranged in regular rows. Of the dermis of animals leather 

 is made. 



The epidermis is a kind of insensible varnish laid over the 

 sensitive skin beneath. It is a tissue composed of dried-up 

 utricles, which form on the surface of the dermis, and which 



