EXCRETORY FUNCTION OF THE SKIN 395 



and filled with an opaque white substance, like soft ointment, which consists 

 of broken-up epithelial cells which have undergone fatty degeneration. Mi- 

 nute capillary vessels overspread them; and their ducts open on either the 

 surface of the skin, close to the hair, or, which is more usual, directly into 

 the follicle of the hair. In the latter case, there are generally two or more 

 glands to each hair, figure 306. 



The story of the structure and development of such epithelial struc- 

 tures as the hair and the nails is best left to the histologist, to whom the 

 student is referred. 



The Excretory Function of the Skin. The function of the skin 

 which is of special interest to this chapter is that of the excretion of the sweat. 

 The fluid secreted by the sweat glands is usually formed so gradually that 

 the watery portion of it escapes by evaporation as fast as it reaches the surface. 

 But during strong exercise, exposure to great external warmth, in some 

 diseases, and when evaporation is prevented, the secretion becomes more 

 sensible and collects on the skin in the form of drops of fluid. 



The perspiration, as the term is sometimes employed in physiology, in- 

 cludes all that portion of the secretions and exudations from the skin which 

 are thrown on the surface by the sweat glands. As a matter of fact, this is 

 mingled with various substances lying on the surface of the skin. The con- 

 tents of the sweat are, in part, matters capable of assuming the form of vapor, 

 such as carbonic acid and water, and in part other matters which are depos- 

 ited on the skin, and mixed with the sebaceous secretions. 



The secretion of the sebaceous glands and hair-follicles consists of cast- 

 off epithelium cells, with nuclei and granules, together with an oily matter, 

 extractive matter, and stearin. In certain parts, also, it is mixed with a 

 peculiar odorous principle, which contains caproic, butyric, and rutic acids. 

 It is similar in composition to the unctuous coating, or vernix caseosa, which 

 is formed on the body of the fetus while in the uterus, and which contains 

 ordinary fat. This sebaceous secretion serves the purpose of keeping the 

 skin moist and supple, and, by its oily nature, of both hindering the evapora- 

 tion from the surface and guarding the skin from the effects of the long-con- 

 tinued action of moisture. But while it thus serves local purposes, its re- 

 moval from the body entitles it to be listed among the excretions of the skin. 



CHEMICAL COMPOSITION OF SWZAT. 



Water 995 



Solids: 5 



Organic acids (formic, acetic, butyric, propionic, 



caproic, caprylic) 0.9 



Salts, chiefly sodium chloride - 1.8 



Neutral fats and cholesterin 0.7 



Extractives (including urea) , with epithelium 1.6 



1000 



