730 COMPOSITION OF SWEA.T. [BOOK n. 



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 the horse, which is singular 

 among hair-covered animals for its frequent profuse sweating, the 

 sweat is said to be always alkaline, and to contain a considerable 

 quantity of some form of proteid. 



In various forms of disease the sweat has been found to 

 contain, 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 hip- 

 puric) 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, respiration 

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

 and in man this cutaneous respiration is, by reason of the thick- 

 ness 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 



