The Skin and the Kidneys 



191 



are fewest in the back and neck, where it is estimated they 

 average 400 to the square inch. They are thickest in the 

 palms of the hands, where they amount to nearly 3000 to 

 each square inch. These minute open- 

 ings occur in the ridges of the skin and 

 may be easily seen with a hand lens. 

 The length of a tube when straight- 

 ened is about one-fourth of an inch. The 

 total number in the body is roughly esti- 

 mated at about 2,500,000, thus making 

 about ten miles of sweat tubes. 

 j 289. The Composition and Quantity of 

 Sweat. The sweat is a turbid, saltish fluid 

 with a feeble but characteristic odor due 

 to certain volatile fatty acids. Of the 

 inorganic constituents of sweat, common 

 salt is the largest and most important. 

 Some carbon dioxide passes out through 

 the skin, but not more than one-fiftieth 

 as much as escapes by the lungs. 



The sweat ordinarily evaporates from 

 the pores into the air as fast as it is 

 formed. This is called insensible perspira- 

 tion. About a pint of sweat is thus lost 

 in the course of a day. 



If there is no perspiration visible we 

 must not infer that the skin is inactive, 

 since sweat is continually passing from 

 the surface. 



^ 290. The Visible Sweat. The visible sweat, frequently 

 spoken of as sensible perspiration, becomes abundant dur- 

 ing active exercise, after copious drinking of cold water, on 

 taking certain drugs, and when the body is exposed to 



FIG. 100. Hair and 

 Hair Follicle. 



A, root of hair ; B, bulb 

 of the hair; C, internal 

 root sheath ; Z>, exter- 

 nal root sheath ; , ex- 

 ternal membrane of 

 follicle; F, muscular 

 fibers attached to the 

 follicle ; H, compound 

 sebaceous gland with 

 its duct, K\ L, simple 

 sebaceous gland; M, 

 opening of the hair 

 follicle. 



