RESPIRATION. 249 



.025 gramme, it is evident that the gross result is a loss of weight to 

 the system, and this loss, by continued respiration, amounts on the 

 average to a little over 70 grammes per day. This is one of the most 

 important facts connected with respiration. It shows that this function 

 is carried on at the expense of the bodily substance, since the oxygen 

 and carbon discharged under the form of carbonic acid weigh more 

 than the oxygen absorbed in a free state. The difference must accord- 

 ingly be supplied in some way by the food ; and if this be withheld, 

 respiration alone will be sufficient to diminish gradually the weight of 

 the body, and to bring it at last to a state of emaciation. 



If we endeavor to ascertain what becomes of the inspired oxygen, 

 it appears, in the first place, that the quantity of this gas which disap- 

 pears from the air is not entirely replaced in the carbonic acid of the 

 breath ; that is, there is less oxygen in the carbonic acid returned to 

 the air by expiration than has been taken from it by inspiration. 



The proportion of oxygen which disappears in the body, over and 

 above that returned by the breath as carbonic acid, varies in different 

 animals. In the herbivora it is about 10 per cent, of the oxygen 

 inspired ; in the carnivora, 20 or 25 per cent. ; and even in the same 

 animal, the proportion of oxygen absorbed, to that of carbonic acid 

 exhaled, varies according to the food upon which he subsists. ID 

 dogs fed on meat, according to Regnault and Reiset,* 25 per cent, 

 of the inspired oxygen disappears in the body of the animal ; but when 

 fed on starchy substances, all but 8 per cent, reappears in the expired 

 carbonic acid. Under some conditions, there may be a difference in 

 the opposite direction ; that is, more oxygen may be contained in the 

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

 phere. In some of the experiments of Regnault and Reiset, with 

 rabbits and fowls fed exclusively on bread and grain, the oxygen in 

 the expired carbonic acid was 101 or 102 per cent, of that taken in by 

 respiration ; and even in man, according to Doyere, the quantity of 

 oxygen discharged as carbonic acid, may be considerably greater than 

 that absorbed. But in general it is the reverse ; the quantity of 

 oxygen not accounted for in the expired carbonic acid being habitually 

 greater in the carnivorous animals than in the herbivora. 



These facts have been established by direct observation, without 

 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 of their significance. The 

 simplest case would be that of an herbivorous animal living exclusively 

 on carbo-hydrates, as starch or sugar. Since these substances already 

 contain oxygen and hydrogen in the proportions to form water, any 

 further oxidation must result in the production of carbonic acid ; and 

 in this case the same quantity of oxygen as that taken in must be 

 returned to the atmosphere as a constituent of the carbonic acid 

 exhaled : the remainder of the carbo-hydrate being separated in the 

 form of water. This process is represented in the following- formula : 

 * Ann ales de Chiniie et de Physique. Paris, 1849, tome zxvi., pp. 409-451. 



