EXCRETION BY THE SKIN 505 



neutral fats, volatile fatty acids, and the merest traces of proteins 

 and urea. It is acid to litmus except in profuse sweating, when it 

 may become neutral or even alkaline. It is secreted by simple 

 gland-tubes, which form coils lined with a single layer of columnar 

 epithelium, in the subcutaneous tissue, with long ducts running up 

 to the surface through the true skin and epidermis. Unless col- 

 lected from the parts of the skin on which there are no hairs, such 

 as the palm, it is apt to be mxed with sebum, a secretion formed 

 by the breaking down of the cells of the sebaceous glands, which 

 open into the hair follicles, and consisting chiefly of glycerin and 

 cholesterin fats, soaps, and salts. Sebum is probably of consider- 

 able importance for maintaining the normal condition of the hair 

 and skin. 



Although it is only occasionally that sweat collects in visible 

 amount on the skin, water is always being given off in the form of 

 vapour. This invisible perspiration leaves behind it on the skin, 

 or in the glands, the whole of the non-volatile constituents, which 

 may be to some extent reabsorbed ; and since even the visible per- 

 spiration is in large part evaporated from the very mouths of the 

 glands in which it is formed, the sweat can hardly be considered a 

 vehicle of solid excretion, even to the small extent indicated by its 

 chemical composition. 



The total quantity of water excreted by the skin, and the relative 

 proportions of visible and invisible perspiration, vary greatly. A 

 dry and warm atmosphere increases, and a moist and cold atmo- 

 sphere diminishes the total, and, within limits, the invisible per- 

 spiration. Visible sweat given the condition of rapid heat-produc- 

 tion in the body as in muscular labour is more readily deposited 

 on freely exposed surfaces when the air is moist than when it is dry. 

 The air in contact with surfaces covered by clothing is never far 

 from being saturated with watery vapour. Here, accordingly, a 

 comparatively slight increase in the activity of the sweat-glands 

 suffices to produce more water than can be at once evaporated; 

 and the excess appears as sweat on the skin., to be absorbed by 

 the clothing without evaporation, or to be evaporated slowly, as 

 the pressure of the aqueous vapour gradually diminishes in con- 

 sequence of diffusion. The power of imbibition (p. 420) of water 

 by the various layers of the skin diminishes as we pass outwards, 

 and the cells of the epidermis are characterized by the rapidity with 

 which they return from a condition of excessive imbibition to their 

 normal state. This constitutes a protective mechanism against 

 excessive loss of water. When the skin is thoroughly moistened, its 

 degree of imbibition is three times the normal. 



The quantity of sweat given off by a man in twenty-four hours 

 varies so much that it would not be profitable to quote here the 

 numerical results obtained under different conditions of tempera- 



