316 



THE SKIN 



the shells of molluscs and the carapaces of turtles. The many parasitic 

 diseases of the skin which are found, such as scabies, tricophytosis, etc., 

 are due to rapidly growing organisms, and are liable to occur especially 

 on skins in which, through infrequency of bathing (which is essentially 

 maceration followed by exfoliation) the outer layers of the epidermis 

 remain unduly long exposed to the environment. 



Sensation or sensitivity is perhaps the next most important function 

 of the human skin. If the term protection be used in a broad sense, 

 much of the usefulness of the skin's sensitivity would come under it. 



FIG. 175 



cM 





a i 



ON- 



Half-schematic section of the skin on the ball of the finger to show the locations of the sense- 

 organs: Sp, layer of papillae; Sr, reticular layer; ON, Ruffini corpuscles; St, nerve-trunk; la, 

 fat globules; cM, Meissner corpuscle; cP, Vater-Pacinian corpuscle; gs, sweat-gland; At, artery. 

 Gold-chloride. (Rauber.) 



One of the chief functions of the dermal sense-organs is to warn the 

 animal within of external injurious conditions such as heat, cold, and 

 various sorts of contact with foreign bodies. Because the sense-organs 

 have functions other than protection, however, we consider them as 

 having uses of their own. The description of these afferent end-organs 

 and of their functions is given in a later chapter (page 351, etc.), so that 

 here we need only mention some of the general conditions of their activity. 

 One of the most basal of biological principles is that of the unity of 



