EXCRETION 417 



quantities of salts, neutral fats, volatile fatty acids, and the 

 merest traces of proteids 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 

 collected from the parts of the skin on which there are no 

 hairs, such as the palm, it is apt to be mixed 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. 



Although it is only occasionally that sweat collects in 

 visible amount on the skin, water is always being given off 

 in the lorm 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 perspiration 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 atmosphere diminishes the total, and, within 

 limits, the invisible perspiration. Visible sweat given the 

 condition of rapid heat-production in the body as in mus- 

 cular 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 consequence of diffusion. 



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