432 THE ELIMINATION OF WASTE PRODUCTS. 



tive proportion of the two kinds of secretion. The secretion of the sebaceous 

 glands appears to be fairly constant, the larger variations of the total sweat 

 depending chiefly on the varying activity of the sweat-glands. Hence, when 

 sweat is scanty, the constituents of the sebum influence largely the characters 

 of the sweat; when, on the contrary, the sweat is very abundant, these may 

 be disregarded, and the sweat may be considered as the product of the sweat- 

 glands. 



We are not able at present to make a complete statement as to what 

 bodies occur exclusively in the sebum and what in the secretion of the 

 sweat-glands. The former consists, very largely, of fats and fatty acids, 

 and appears to contain some form or forms of proteids ; but we have 

 reason to think that the sweat-glands secrete, in some quantity, some 

 forms of fat, and especially volatile fatty acids. 



When sweat is scanty, the reaction is generally acid, but when abund- 

 ant, is alkaline ; and when a portion of the skin is well washed the 

 sweat which is collected immediately afterward is usually alkaline. From 

 this we may infer that the secretion of the sweat-glands is naturally 

 alkaline, but that when mixed, sweat is acid ; the acidity being due to fatty 

 (or other) acids of the sebum. In the horse, which is singular in hair- 

 covered animals for its frequent profuse sweating, the sweat is said to be 

 always acid and to contain a considerable quantity of some form of 

 proteid. These features are probaby due to the large admixture of 

 sebum from the numerous sebaceous glands connected with the hairs. 



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 per cent., 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 odor ; it is 

 probable that many various volatile acids are present in small quanti- 

 ties. 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 palm of the hand, 

 where sebaceous glands are not present. (4) The evidence goes to show 

 that neither urea nor any ammonia compound exists in the normal secre- 

 tion to any extent, though some observers have found a considerable quan- 

 tity of urea (calculated at 10 grms. in the twenty-four hours for the 

 whole body). Apparently a small amount of nitrogen leaves the body 

 through the skin, but this is probably supplied by the sebum or by the 

 epidermis. 



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

 times 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 sweat when taken 

 internally as medicines. 



Cutaneous Respiration. 



367. A frog whose lungs have been removed will continue to live for 

 some time ; and during that period will continue, not only to produce car- 

 bonic 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 



