608 MANUAL OF HISTOLOGY. 



as high as 550 to 1 500. We see, then, that the excretion of water through 

 the kidneys ( 274) exceeds that of the skin in general, and further, that 

 but a small amount of the products of decomposition escape through the 

 latter. The amount of moisture eliminated by the skin is, however, far 

 more than that exhaled from the surface of the lungs, which is only from 

 5 to 7 hundred grammes in twenty-four hours. Further details on this 

 subject may be left to physiologists.. 



The chemical analyses which have been hitherto made of the watery 

 secretions of the skin have been directed partly to the fluid exhaled over 

 the surface of the body, and condensed there in drops again ; partly to 

 the water welling from the perspiratory glands themselves in the form of 

 drops ; and lastly, towards both together. To this mixture, then, the term 

 sweat is generally applied. 



The sweat (sudor) always contains an admixture of cast-off epithelial 

 scales and fatty molecules, which last are derived partly from the seba- 

 ceous matter of the skin, partly from the contents of the convoluted 

 glands. Otherwise the secretion is quite devoid of form elements of 

 every kind. 



It is a clear, colourless fluid, normally of acid reaction when fresh, but 

 becoming, after some time, neutral or even alkaline. Its taste is usually 

 salty, and smell more or less intense, and dependent on the presence of 

 volatile fatty acids. 



The proportion of solid constituents is small but variable, decreasing 

 relatively with the amount of water excreted : it may be stated at 0'5-2 

 per cent, of the latter. They consist of both organic and mineral matters. 

 To the first belong several members of the volatile fatty acid series (p. 

 24), and prominent among them formic, buttyric, and acetic acids. The 

 presence also of metaceton, capronic, caprylic t and caprinic acids is at 

 least probable. But there can be no doubt that we have here to deal 

 with a very variable fluid, which is indicated, among other things, by the 

 different odour of the perspiration of various parts of the body, as well as 

 of different races of men, such as that of Europeans and negroes. Accord- 

 ing to Favre's observations, further, a peculiar acid may be demonstrated 

 in sweat, which he has named hydrotinic acid (p. 36). 



Recent observation has shown also that sweat, even under normal 

 conditions, contains urea, to the presence of which substance the rapid 

 change in the reaction of the secretion, combined with the development of 

 ammonia, may be ascribed. In morbid conditions of the system also, 

 induced by impairment of the functions of the kidneys, this compound 

 may be excreted largely by the skin. Among the other widely diffused 

 animal bases none have been demonstrated here as yet. 



Neutral fat is likewise a constant constituent, and cliolestearin has 

 also been met with (ScJwttin). Leube also met with a peculiar albu- 

 minous substance here. 



Under abnormal conditions the biliary pigments may also appear in the 

 sweat. 



The mineral matters found here consist, besides iron and phosphate of 

 calcium (derived from the epithelial cells), principally of chlorides of the 

 alkalies, and above all of sodium ; then, again, small quantities of phos- 

 phates and sulphates of the alkaline salts. Finally, free carbonic acid is 

 met with here, and salts of ammonia, due to decomposition. 



