244 FUNCTIONS OF NUTRITION. 



spiration. Morin* found that the urine always contains in solution 

 certain gases, of which carbonic acid is the most abundant. The mean 

 result of fifteen observations showed that urine excreted during, the 

 night contains about 1.96 per cent, of its volume of carbonic acid. During 

 the day the quantity of this gas varies considerably, according to the 

 condition of repose or activity ; since after remaining quiet for an hour 

 or two, it was only 1.19 per cent, of the volume of the urine, while 

 after continued exertion for a similar period the urine was augmented 

 in quantity, and its proportion of carbonic acid at the same time nearly 

 doubled, amounting to 2.29 per cent, of its volume. 



An equal or even greater activity of gaseous exhalation takes place 

 by the skin. It has been found, by inclosing one of the limbs in an 

 air-tight case, that the air in which it is confined loses oxygen and gains 

 carbonic acid. From an experiment of this sort, Scharling estimated 

 the carbonic acid given off from the whole cutaneous surface, in man, 

 as from one-sixtieth to one-thirtieth of that discharged by the lungs. 

 In the more recent observations of Aubert, the whole body without 

 clothing, was confined in an air-tight case, leaving only the head 

 exposed. Ventilation was kept up during the experiment with air 

 free from carbonic acid, while the carbonic acid exhaled from the 

 body was absorbed by baryta-water. Each observation lasted for two 

 hours, and the average result obtained was that, for the entire day 

 of twenty-four hours, 198 cubic centimetres of carbonic acid were 

 exhaled from the skin ; a quantity representing rather less than one 

 two-hundredth of that given off by the lungs. 



In the amphibious reptiles, as frogs, newts, and salamanders, which 

 breathe by lungs, and yet can remain under water for a considerable 

 time, the integument takes a more active part in respiration. The 

 skin of these animals, which is thin, moist, and covered with a deli- 

 cate epithelial layer, presents the most favorable conditions for gaseous 

 transudation ; and beneath the surface of the water, while the lungs 

 are comparatively inactive, exhalation and absorption take place through 

 the skin, and respiration goes on almost without interruption. 



Indifference of Nitrogen in the Act of Respiration. Notwith- 

 standing the abundance of free nitrogen in the atmosphere, and its 

 existence to some extent in the circulating fluids, this substance takes 

 no direct part in respiration or nutrition. Even in vegetables, the 

 nitrogen required for their albuminous ingredients is derived only from 

 pre-existing nitrogenous compounds, mainly nitrates and ammonium 

 salts. In animals, according to the conclusions generally accepted, f 

 there is no satisfactory evidence that the free nitrogen of the air has 

 any share in the phenomena of combination or decomposition within 

 the body. It appears to serve as a vehicle or medium of admixture 

 for the introduction of oxygen ; remaining in other respects an indif- 

 ferent substance in the respiratory process. 



* Journal de Pharmacie et de Chimie. Paris, 1864, tome xlv., p. 396. 

 f Hoppe-Seyler, Physiologische Cheraie. Berlin, 1877, p. 48. 



