252 EESPIRATION. 



contrary to what happens in the male, 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 exhaled increases in a notable manner ; then it de- 

 creases again, as in the male, as the woman advances toward old age. 



6. During the whole period of pregnancy, the exhalation of car- 

 bonic 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. 



Prof. Scharling, in a similar series of investigations, 1 found that 

 the quantity of carbonic acid exhaled was greater during the diges- 

 tion of food than in the fasting condition. It is greater, also, in the 

 waking state than during .sleep; and in a state of activity than in 

 one of quietude. It is diminished, also, by fatigue, and by most 

 conditions which interfere with perfect health. 



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

 but the interchange of gases takes place, also, to some extent through 

 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 in carbonic acid. By an experiment of this sort, performed by 

 Prof. Scharling, 2 it was ascertained that the carbonic acid given off 

 from the whole cutaneous surface, in the human subject, is from 

 one-sixtieth to one-thirtieth of that discharged during the same 

 period from the lungs. In the true amphibious animals, that is, 

 those which breathe by lungs, and can yet remain under water for 

 an indefinite period without injury (as frogs, and salamanders), the 

 respiratory function of the skin is very active. In these animals, 

 the integument is very vascular, moist, and flexible ; and is covered, 

 not with dry cuticle, but with a very thin and delicate layer of 

 epithelium. It, therefore, presents all the conditions necessary for 

 the accomplishment of respiration ; and while the animal remains 

 beneath the surface, and the lungs are in a state of inactivity, the 

 exhalation and absorption of gases continue to take place through 

 the skin, and the process of respiration goes on in a nearly unin- 

 terrupted manner. 



1 Annales de Chirai et <\e Pharmacia, vol. viii. p. 490. 



2 In Carpenter's Human Physiology, Philaria. ed., 1855, p. 308. 



