CHAP, xvi.] PHYSIOLOGICAL OFFICE OF RESPIRATION. 243 



pulmonary cells, is in contact with a gaseous mixture of which 

 the composition is obviously always the same. The exchange of 

 gases can, then, be effected regularly without interruption, and 

 even without any sudden variation. 



The lungs receive the exterior air, that is to say, a gaseous 

 mixture of about 21 of oxygen and 79 of azote, besides a small 

 quantity of vapour of water and about four ten- thousandths of 

 carbonic acid. They restore to the atmosphere air remarkably 

 impoverished of oxygen, since it scarcely contains more than 

 18 in 100 (Duges), but impregnated instead with carbonic acid 

 and the vapour of water. 



From numerous observations, among which we must first cite 

 those of Lavoisier, next those of Regnault and Reiset, 1 it is 

 proved that the quantity of oxygen contained in the carbonic 

 acid exhaled by the lungs is sensibly inferior to the quantity of 

 atmospheric oxygen absorbed. Lavoisier had already noted this 

 interesting fact. MM. Eegnault and Reiset saw that this pro- 

 portion varied very little in the same animal species, living under 

 normal condition^. The proportion between the oxygen of the 

 carbonic acid exhaled and the oxygen absorbed has been, accord- 

 ing to their observations, from 0'743 to 0750 in the dog, 920 

 in the rabbit, &c. Atmospheric oxygen indeed ought to be 

 considered as a true aliment ; it combines with the plasmas, 

 with the matter of the anatomical elements, in a word, with the 

 living substance ; it enters into numerous compounds, and is 

 eliminated through diverse channels. The organism only takes 

 from the atmosphere that quantity of oxygen which is necessary 

 for it : it rations itself. In an atmosphere containing 40 and 

 even 60 in 100 of oxygen, the absorption of this gas is not 

 sensibly more considerable than in the common air, unless there 

 is variation in the regimen. In this last case, on the contrary, 

 the quantity of oxygen absorbed varies ; for there must be more 

 or less of it, according to the variation in the proportion and 

 the nature of the substances to be assimilated in the sanguineous 



1 Annales de Chimie et de Physique, 2 serie, t. XXVI. 



R 2 



