286 RESPIRATION. 



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

 time, the thin, moist, and flexible integument takes a still more active 

 part in the process of respiration. The skin in these animals is covered, 

 not with dry cuticle, but with a delicate layer of epithelium. It accord- 

 ingly presents all the conditions necessary for the accomplishment of 

 respiration ; and while the animal remains beneath the surface of the 

 water, though the lungs are in a state of comparative inactivity, the 

 exhalation and absorption of gases continue to take place through the 

 skin, and respiration goes on without interruption. 



Relation between the Oxygen absorbed in respiration and the Car- 

 bonic Acid given off. It has been seen that, in the human subject, with 

 each respiration, on the average, 16 cubic centimetres of oxygen are 

 taken into the system by absorption, and 13 cubic centimetres of car- 

 bonic acid given off. As the oxygen thus taken in weighs rather less 

 than .023 gramme, while the carbonic acid discharged weighs .025 

 gramme, it is evident that the gross result of the process is a loss of 

 weight to the system, and this loss of substance by continued respira- 

 tion amounts on the average to a little over 70 grammes per day. This 

 is one of the most important facts connected with the plvysiology of 

 respiration. It shows that this function is carried on at the expense of 

 the substance of the animal body, since the oxygen and carbon dis- 

 charged under the form of carbonic acid, weigh more than the oxygen 

 which is absorbed in a free state. This difference in quantity must 

 accordingly be supplied in some way by the ingredients of the food; 

 and if this be withheld, the progress of respiration alone will be suffi- 

 cient to diminish gradually the weight of the body, and to bring it to a 

 state of more or less complete emaciation. 



If we endeavor to ascertain what becomes of the oxygen itself, it is 

 found that the quantity of this gas which disappears from the inspired 

 air is not entirely replaced in the carbonic acid exhaled ; that is, there 

 is less oxygen in the carbonic acid which is returned to the air by expi- 

 ration than has been lost by it during inspiration. 



The proportion of ox}-gen which disappears in the interior of the 

 body, over and above that returned in the breath under the form of 

 carbonic acid, varies in different kinds of animals. In the herbivora it 

 is about 10 per cent, of the whole amount of oxygen inspired; in the 

 carnivora, 20 or 25 per cent.; and even in the same animal, the propor- 

 tion of oxygen absorbed, to that of carbonic acid exhaled, varies accord- 

 ing to the kind of food upon which he subsists. In clogs, while fed on 

 meat, according to the experiments of Regnault and Reiset, 1 25 per 

 cent, of the inspired oxygen disappeared in the body of the animal ; 

 but when fed on starchy substances, all but 8 per cent, reappeared in 

 the expired carbonic acid. Under some circumstances, a difference may 

 show itself in the opposite direction ; that is, more oxygen may be con- 

 tained in the carbonic acid exhaled than is absorbed in a free state from 



1 Annales de Chimie et de Physique, tome xxvi. p. 428. 



