696 COMPOSITION OF SWEAT. [BOOK n. 



Taking ordinary sweat, such as may be obtained by enclosing 

 the arm in a bag, we may say that, in man, the average amount of 

 solids is from 1 to 2 p. c., of which about two-thirds consist of 

 organic substances. The chief normal constituents are : (1) Sodium 

 chloride, with small quantities of other inorganic salts. (2) Various 

 acids of the fatty series, such as formic, acetic, butyric, with 

 probably propionic, caproic, and caprylic. The presence of these 

 latter is inferred from the odour ; it is probable that many various 

 volatile acids are present in small quantities. Lactic acid, which 

 has been reckoned as a normal constituent, is stated not to be 

 present in health. (3) Neutral fats, and cholesterin; these have 

 been detected even in places, such as the palms of the hand, where 

 sebaceous glands are absent. (4) The evidence goes to shew that 

 neither urea nor any ammonia compound exists in the normal 

 secretion to any extent, though some observers have found a con- 

 siderable quantity of urea (calculated at 10 grms. in the 24 hours 

 for the whole body). Apparently some small amount of nitrogen 

 leaves the body by the skin as a whole, but this is probably 

 supplied by the sebum or by the epidermis. 



In various forms of disease the sweat has been found to contain, 

 sometimes in considerable quantities, blood, albumin, urea (par- 

 ticularly in cholera), uric acid, calcium oxalate, sugar (in diabetic 

 patients), lactic acid, indigo (or indigo-yielding bodies giving rise 

 to ' blue ' sweat), bile and other pigments. Iodine and potassium 

 iodide, succinic, tartaric, and benzoic (partly as hippuric) acids 

 have been found in the sweat when taken internally as medicines. 



Cutaneous Respiration. 



439. A frog, whose lungs have been removed, will continue 

 to live for some time ; and during that period will continue 

 not only to produce carbonic acid, but also to consume oxygen. 

 In other words, the frog is able to breathe without lungs, respi- 

 ration being carried on efficiently by means of the skin. In 

 mammals and in man this cutaneous respiration is, by reason of 

 the thickness of the epidermis, restricted to within very narrow 

 limits; and indeed it has been questioned whether it can be 

 spoken of at all as a true respiration. When the body remains 

 for some time in a closed chamber to which the air passing in 

 and out of the lungs has no access (as when the body is enclosed 

 in a large air-tight bag fitting tightly round the neck, or where a 

 tube in the trachea carries air to and from the lungs of an animal 

 placed in an air-tight box), it is found that the air in the chamber 

 loses oxygen and gains carbonic acid. The amount of carbonic 

 acid which is thus thrown off by the skin of an average man in 

 24 hours amounts to about 10 grms., or according to some observers 

 to (no more than) about 4 grms., increasing with a rise of tempe- 

 rature, and being very markedly augmented by bodily exercise. 



