552 COMPOSITION OF SWEAT. [BOOK n. 



consist of organic substances. The chief normal constituents 

 are: (1) Sodium chloride, with small quantities of other inor- 

 ganic 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 pres- 

 ent 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 considerable quan- 

 tity of urea (calculated at 10 grms. in the 24 hours for the whole 

 body). Apparently some small amount of nitrogen leaves the 

 bod} r 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 con- 

 tain, sometimes in considerable quantities, blood, albumin, urea 

 (particularly 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 inter- 

 nally as medicines. 



Cutaneous Respiration. 



351. 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 temperature, and being very markedly augmented by 

 bodily exercise. It is stated that the amount of oxygen con- 



