CHANGES IN THE AIR BY RESPIRATION. 287 



the atmosphere. In some of the experiments of Regnault and Reiset, 1 

 where rabbits and fowls had been led exclusively upon bread and grain, 

 the proportion of oxygen in the expired carbonic acid was 101 or 102 

 per cent, of that taken in by respiration ; and even in the human sub- 

 ject, according to the observations of Doyere, the quantity of oxygen 

 eliminated by the breath as carbonic acid, may be considerably greater 

 than that absorbed. But, as a general rule, it is the reverse ; the quan- 

 tity of oxygen which is not to be accounted for in the expired carbonic 

 acid being habitually greater in the carnivorous animals than in the 

 Jierbivora. 



These facts have been established by direct observation, and without 

 any reference to the supposed manner in which the internal changes of 

 respiration take place. Nevertheless, they are susceptible of so ready 

 an explanation that there can be little doubt as to their significance. 

 The simplest case for examination would be that of an herbivorous ani- 

 mal living exclusively upon the carbo-hydrates, as starch or sugar. 

 Since these substances, as their name implies, already contain hydrogen 

 and oxygen in the proportions to form water, any further oxidation 

 which they undergo must result in the production of carbonic acid ; 

 and in this case exactly the same quantity of oxygen as that taken in 

 must necessarily be returned to the atmosphere as a constituent of the 

 carbonic acid exhaled ; the remainder of the substance being separated 

 from its combinations in the form of water. This process is represented 

 in the following formula : 



Starch. Carbonic acid. Water. 



C 6 H 10 0- + 12 = C G 12 + H 10 5 . 



In an animal supported upon this food, therefore, the whole of the 

 oxygen taken in by respiration would reappear in the expired carbonic 

 acid. But in an animal feeding also upon fatty substances, the propor- 

 tions would be changed. As these matters no longer contain oxj^gen 

 in the requisite quantity to form water with the hydrogen present, more 

 oxygen must be taken in with the breath than is sufficient to unite with 

 the carbon under the form of carbonic acid ; and consequently a portion 

 of it will disappear from the gaseous products of respiration. The 

 change in this instance is as follows : 



Oleine. Carbonic acid. Water. 



Cor^oA H- 160 = C 37 1U + H 104 52 . 



In effecting, therefore, the complete disappearance of a fatty sub- 

 stance, 160 parts of oxygen will be absorbed, and only 114 parts re- 

 turned in the form of carbonic acid. This will also take place where 

 albuminous matters are used as food, since it is known that all the 

 nitrogen of these substances is expelled from the body under the form 

 of urea ; and after the separation of urea from albumen, a body must be 

 left which is analogous in composition to fat ; that is, which contains 



1 Annales de Chimie et de Physique, tome xxvi. pp. 409-451. 



