EXCRETION. 435 



that the rate of secretion varies readily with variations in tempera- 

 ture, 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 inorganic 

 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 im- 

 portant as an excrementitious fluid as it is as a means for the regula- 

 tion 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 totaljirea varies from 16 to 20 feet 

 in man and from 12 to 16 feetlrrivbriianT It varies in thickness in 

 different localities of the body from i to T ^-Q of an inch. The skin 

 consists of two principal layers: viz* r ^ deep layer, the derma or 

 corium, and a superficial layer, the epidermis. 



The derma or corium may be suMlVfcTed into a reticulated and a 

 papillary 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 consists mainly of club-shaped elevations 

 or projections of the amorphous matter constituting the papillas. 

 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. 199). 



The epidermis is an extra- vascular structure, con^isjtingjejitirely 

 of epithelial cells. It may also be subdivided into two layers 

 the Malpighian or pigmentary layer, and the corneous or horny layer. 

 The former is closely applied to the papillary layer of the true skin 



