110 ZOOLOGY. 



harden only by being exposed to the air. It is composed of 

 several layers, of which the lower or deepest layer, being soft 

 and containing the pigmentary matter giving the colour to 

 the skin, has been viewed and described by some physiologists 

 as a distinct membrane, under the name of retemucosum. In 

 man, and in many animals resembling him, the epidermis is 

 cast off in the form of small scales, which are constantly 

 renewed. In serpents, the entire epidermis is cast off as a 

 slough. 



The pores of the skin correspond to the summits of the 

 papillse just described. They give passage to the perspiration 

 or sweat, an acid liquid formed by secretion, and which must 

 not be confounded with the insensible perspiration. These 

 pores, exceedingly minute, do not traverse the skin ; they are 

 merely the orifices of the excretory canals of so many small 

 ampullae lodged in the substance of the skin, and which are 

 the secreting organs of the sweat. 



Other larger orifices are found on the surface of the skin ; 

 some for the passage of the hairs, and others for the escape of 

 a fatty matter secreted by follicles lodged in the substance of 

 the skin. These are called sebaceous glands or follicles. 



208. The principal use of the epidermis is to throw 

 obstacles in the way of evaporation, and to protect the sensi- 

 tive skin beneath. It deadens more or less all impressions, 

 and hence its thickness on the heel and sole of the foot 

 generally : it thickens whenever it is exposed to friction. 

 Finally, in some animals it becomes encrusted with calcareous 

 matters, becoming then altogether inflexible, and rendering 

 the surface of the body insensible. 



209. The sensibility of the skin resides in the dermis 

 and depends on the nerves of touch distributed to the 

 papillae. 



210. Special Organs of Touch. Tactile sensibility is 

 spread over the whole body, but it is in the extremity of the 

 fingers alone that the true power of touch resides. The hand, 

 of course, is especially made for the exercise of this faculty. 

 The delicacy of the integuments, the length of the fingers, 

 and the opposing thumb, all contribute to this effect. 



In most animals these organs are not so favourably 

 arranged : nevertheless, the proboscis of the elephant is a 

 wonderful tactile instrument. There are animals which em- 

 ploy the tongue for this purpose, and others are provided with 

 palpi, tentacula, &c. (Figs. 9, 10). 



