ix THE SKIN AND CUTANEOUS GLANDS 493 



The much lower concentration of the sweat obtained by vapour 

 baths is evidently due to its dilution by the water condensed 

 from the steam. 



There is a marked difference between these results and those 

 obtained by Argutinsky, since 34 per cent of the total nitrogen 

 consists of urea-nitrogen and 7'5 of ammonia-nitrogen. The rest 

 is accounted for by traces of protein and other nitrogenous 

 substances, including uric acid, which was demonstrated by the 

 murexide test in experiments 1, 2, 4 of the table. 



Just as the nature of the solid substances eliminated by the 

 sudoriferous glands in the form of sweat shows the function of the 

 skin to be subsidiary to, and in a certain measure vicarious of, that 

 of the kidneys, so the insensible output of water and carbonic acid 

 with simultaneous absorption of oxygen, indicates that the skin 

 is a respiratory surface, aiding and partly supplementing the 

 function of the lungs. In man and mammals, however, the 

 gaseous exchanges by the skin are insignificant in comparison with 

 those of the lungs. According to Aubert and Lange, the total 

 excretion of carbonic acid by the human skin is only 3-4 grms. 

 per diem, but this figure was doubled in the later work of 

 Schierbeck. 



The small amount of C0 2 given off by the skin is not, as some 

 hold, due to the thickness of the epidermis. In all probability 

 there is no sensible respiratory gas exchange through the stratum 

 corneum, but this, like the excretion of water, is entirely accom- 

 plished by the ducts of the cutaneous glands. The scanty 

 secretion of C0 2 is more likely due to the fact that the skin is 

 irrigated by arterial blood, in which the carbonic acid is at low 

 tension. In certain amphibia, on the contrary, e.g. frogs, the 

 gaseous exchange that takes place by the skin is (in consequence 

 of the limited pulmonary surface, the habitual moisture of the 

 skin, and above all the fact that, like the lungs, it is supplied with 

 mixed blood from the single ventricle of the heart) in excess of 

 that by the lungs, so that these animals live for a long time after 

 the lungs have been cut out. 



In man it is doubtful whether the excretion of carbonic acid 

 by the skin and the (usually smaller) absorption of oxygen occur, 

 as in the lungs, principally by a process of diffusion. The former 

 (as was rightly observed by Foster) might not be derived directly 

 from the blood, but from decomposition of the carbonates con- 

 tained in the sweat ; in the same way the oxygen that disappears 

 may not be absorbed by the blood circulating in the skin, but may 

 be utilised to oxidise some of the organic constituents of the sweat. 



From this point of view the results of Schierbeck's experiments 

 (1893) are interesting. He studied the effect on human cutaneous 

 secretion of different degrees of external temperature, varying from 

 29 to 39 C., and found that the excretion of C0 2 does not vary 



