332 RESPIRATION. 



jected to any considerable fluctuations. Although the advantages 

 of this method are striking, it is not entirely free from objections, 

 for the saturation with aqueous vapour of the air which the animal 

 is inspiring, and the increased amount of nitrogen in the air after 

 the long-continued respiration of animals within a limited space, 

 are elements which must speedily exert some influence on the 

 respiratory process. 



When we pass in review the most general results yielded by 

 different investigations on the interchange of gases in the lungs, 

 we find in the first place, that the blood in the lungs gives off car- 

 bonic acid and aqueous vapour to the inspired air, and takes up 

 oxygen from the latter ; a very small quantity of nitrogen also 

 commonly passes from the blood into the respired air, although 

 under special conditions, the opposite sometimes occurs. The 

 first question which presses itself upon our notice is undoubtedly 

 the determination of the relation existing between the exhaled car- 

 bonic acid and the oxygen from the inspired air which has dis- 

 appeared in the lungs. It is well known that the volume of 

 carbonic acid gas is equal to the volume of the oxygen contained 

 in it ; if, therefore, a volume of carbonic acid were found in the 

 expired air, which was equal to that of the oxygen which had dis- 

 appeared from the inspired air, we might be led to conclude with 

 many of the older inquirers, that the oxygen absorbed in the pul- 

 monary vesicles is exactly sufficient to form the carbonic acid which 

 they exhaled. This, however, is by no means the case, for under 

 ordinary relations, the volume of oxygen absorbed is much larger 

 than the volume of carbonic acid which is exhaled. The oxygen 

 does not therefore serve merely for the oxidation of the carbon, 

 but also for that of the hydrogen of the animal constituents. If, 

 for instance, animals be allowed to breathe in an inclosed space, 

 and we then analyse the air which has been modified by their 

 respiration, we find that more free oxygen has disappeared than 

 could have been employed in the formation of the carbonic acid 

 contained therein ; we obtain the same result if, as was done by 

 Marchand, we compare the loss of weight in the animal during 

 the period of the experiment, with the oxygen contained in the 

 expired air, and distributed in the carbonic acid and aqueous 

 vapour; for in this case we find that the animal has lost less in 

 weight than might be expected from the quantities of excreted 

 carbon and hydrogen ; consequently a substance appreciable by 

 weight must be absorbed by the body of the animal during respi- 

 ration, and this can be no other than oxygen, as the quantities 



