472 TEXT-BOOK OF PHYSIOLOGY 



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 J to y^ of an inch. 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. 



The Sweat-glands. These glands are tubular in shape, the inner 

 extremity of each being coiled upon itself a number of times, forming a 

 little ball situated in the derma or the subcutaneous connective tissue. From 

 this coil the duct passes up in a straight direction to the epidermis, where 

 it makes a few spiral turns, after which it opens obliquely on the surface. 

 The gland consists of a basement membrane lined with epithelial cells. 

 It is supplied abundantly with blood-vessels and nerves. The sweat- 

 glands are extremely numerous all over the cutaneous surface, though 

 they are more thickly disposed in some situations than others. They 

 probably average 400 to the square centimeter; the total number has been 

 estimated at from 2,000,000 to 2,500,000. 



The Influence of the Nerve System on the Production of Sweat. 

 The secretion of sweat, though a product of the activity of epithelial cells 

 and dependent on a variety of conditions, is regulated to a large extent by 



