476 TEXT-BOOK OF PHYSIOLOGY 



at from 700 to 1000 grams; the exact amount, however, is difficult of determi- 

 nation, for the reason that the rate of secretion varies greatly with variations in 

 temperature, food, drink, season of the year, etc. 



Chemic analysis of the sweat shows that it contains but from 0.5 to 2.5 

 per cent, of solid constituents, the variation in the percentage depending on 

 the quantity of water secreted. The solids consist of traces of urea, neutral 

 fats, lactic and sudoric acids in combination with alkaline bases, and inorganic 

 salts (Fovel). Other observers, however, have not been able to detect the 

 presence of either lactic or sudoric acid. Urea is a constant ingredient, 

 though its percentage is extremely small, possibly not more than o.i per 

 cent. The amount, however, may be very much increased in uremic 

 conditions, the result of acute or chronic disease of the kidneys. The inor- 

 ganic constituents consist mainly of sodium chlorid and alkaline and 

 earthy phosphates. Carbonic acid is also present in the free state as well 

 as in combination with alkaline bases. 



The very small quantity of the solid constituents in the sweat, taken in 

 connection with the fact that it is excreted most abundantly when the external 

 temperature is high, indicates that it is not so important as an excrementi- 

 tious fluid as it is as a means for the regulation of the temperature of 

 the body. 



The sweat is a product of the secretory activity of specialized glands, 

 the sweat-glands, embedded in the skin, to the histologic structures of 

 which they bear a special relation. 



THE SKIN 



The skin is a complexly organized structure investing the entire external 

 surface of the body. Its total area varies from 1.17 to 1.35 square meters in 

 man and from i.i to 1.17 square meters in woman. It varies in thickness 

 in different localities of the body from 3 to 0.25 of a millimeter. The skin 

 consists of two principal layers: viz., a deep layer, the derma or corium, 

 and a superficial layer, the epidermis. 



The derma or corium may be subdivided into a reticulated and a pap- 

 illary layer. The reticulated layer consists of white fibrous and yellow 

 elastic tissue, non-striated muscle-fibers, woven together in every direction 

 and forming an areolar network, in the meshes of which are deposited 

 masses of fat and a structureless amorphous matter; the papillary layer con- 

 sists mainly of club-shaped elevations or projections of the amorphous 

 matter constituting the papillae. The reticulated layer serves to connect 

 the skin with the underlying structures and to afford support for the blood- 

 vessels, nerves, and lymphatics which are distributed to the papillae (Fig. 225). 



The epidermis is an extra-vascular structure consisting entirely of epi- 

 thelial cells. It may also be subdivided into two layers the Malpighian or 

 pigmentary layer, and the corneous or horny layer. The former is closely ap- 

 plied to the papillary layer of the true skin and is composed of large nucle- 

 ated cells, the lowest layer of which, the "prickle cells," contains the pig- 

 ment granules which give to the skin its varying hues in different individu- 

 als and in different races of men; the corneous layer is composed of flattened 

 cells which from their exposure to the atmosphere, etc., are hard and horny 

 in texture. 



