CHANGES IN THE AIR BY RESPIRATION. 285 



to the same law as in the male, from the age of eight years until puberty. 

 But at the period of puberty, at the appearance of menstruation, the 

 exhalation ceases to increase ; and it afterward remains stationary so 

 long as the menstrual periods recur with regularity. At the cessation 

 of the menses, the quantity of carbonic acid increases in a notable 

 manner ; then it decreases again, as in the male, toward old age. 



6. During the whole period of pregnancy, the exhalation of carbonic 

 acid rises, for the time, to the same standard as in women whose menses 

 have ceased. 



7. In both sexes, and at all ages, the quantity of carbonic acid is 

 greater, as the constitution is stronger and the muscular system more 

 fully developed. 



The process of respiration is not altogether confined to the lungs, 

 but the discharge of carbonic acid takes place also, to a slight extent, 

 both by the urine and the perspiration. Morin 1 has found that the 

 urine always contains gases in solution, of which carbonic acid is con- 

 siderably 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 contained in the urine varied considerably, according to the 

 condition of muscular 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 the same space of time, not only was 

 the urine augmented in quantity, but the proportion of carbonic acid 

 contained in it was 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, Prof. Scharling esti- 

 mated that the carbonic acid given off from the whole cutaneous surface, 

 in man, is from one-sixtieth to one-thirtieth of that discharged during 

 the same period from the lungs. In the more recent and complete obser- 

 vations of Aubert upon this subject, the whole body without clothing 

 was confined in an air-tight case, leaving only the head exposed. A 

 continuous ventilation of the space was kept up during the course 

 of the experiment with air free from carbonic acid, while the carbonic 

 acid exhaled from the surface of 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 0.5 per cent, of that given off by the lungs 

 in the same time. 



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



1 Recherches sur les Gaz libres de 1'Uriue. Journal de Pbarmacie et de Chinrie. 

 Paris, 1864, tome xlv. p. 396. 



