136 BIOLOGY. [Book ii. 



of the complete combustion of a small quantity of albuminoidal 

 matters.^ 



TJie exhaled oxygen also no doubt is derived from the outer 

 air. It is an overplus which cannot be utilized. So far, at 

 least, it has not been shown that the chemical reactions of 

 nutrition set at liberty oxygen gas. 



As to the carbonic acid abundantly exhaled from the general 

 surface of the bodies of animals, and especially through their 

 respiratory organs, it is almost wholly derived from the slow 

 combustion, the oxydation, of the tissues. In effect, we cannot, 

 in striking the nutritive animal balance, place any value upon 

 the small proportion of carbonic acid contained in the atmosphere. 

 The carbonic acid exhaled by the animal is a tiTie principle of 

 dis-assimilation ; formed in the very substance of the anatomical 

 elements, it is first sent into the circulatory torrent of sanguiferous 

 animals, to be finally restored to the aerian medium. The inmost 

 mode of formation of the carbonic acid is besides far from be- 

 ing well known. Thus the quantity of carbonic acid exhaled by 

 the lungs does not exactly correspond to the quantity of oxygen 

 absorbed. It seems then that we have not here a simple question 

 of combustion.^ ' It has been possible to compare, in the various 

 tissues and organs, the power of the formation of carbonic acid, 

 that is to say, to estimate, in a certain degree, the relative 

 energy of oxydation in the superior animals. The parenchymas 

 of certain very active glands, such as the liver and the kidneys, 

 yield a strong proportion of carbonic acid. The muscular tissue 

 furnishes less of it, and the nervose tissue less still. 



It is not in the blood that the greater part of the exhaled 

 carbonic acid is produced, but in the histological elements them- 

 selves ; also we find that the blood has always a lower temperature 

 than the tissues from which it proceeds. 



As a mineral substance, and an exhaled liquid, we have to 

 mention water, excreted specially through the skin and kidneys 



' J. Gavarret, Des Phinomines Physiques cle la Vie, pp. 177, 178. 



' 01. Bernard, Progres et Marche de la Physiologie Genlrale en Frcmce, p. 191. 



