MAMMALS. BIRDS. 



367 



observers in the value and importance of their results. When we 

 consider, in reference to the mammalia, what influence the different 

 food on which they live may have upon the quantitative relations 

 of the interchange of gases in the lungs, we find, on referring to 

 the remarks already made in relation to this subject, that the 

 differences which have been observed in the respiratory relations 

 of the herbivora and carnivora, do not depend upon any difference 

 in their organization, but are almost wholly referrible to the 

 influence of the food upon which they subsist. For in the same 

 manner as we observe that the urine of the carnivora, when fed 

 upon vegetables, is similar to, if not identical with, that of the 

 herbivora, and that the urine of herbivorous animals living on 

 animal substances is analogous to that of the carnivora, we also 

 find that carnivorous animals, which live principally on amylaceous 

 matters, exhibit the same respiratory relations as the herbivora, 

 and conversely. This fact has been proved beyond a doubt by the 

 careful investigations of Regnault and Reiset, and more recently 

 by Bidder and Schmidt. We subjoin a table of these relations 

 as they are given by Regnault and Reiset. We have intro- 

 duced it here merely by way of furnishing a general retrospect of 

 the whole : 



The absolute quantity of absorbed oxygen and exhaled carbonic 

 acid is, however, somewhat fluctuating, which partially explains the 

 great discrepancy observable between these results and the numbers 

 obtained by other observers in their experiments on rabbits and 

 dogs. On instituting a comparison between the numbers ob- 

 tained by different investigators, we find that the results coincide in 

 this respect, that the carnivora, when kept upon their ordinary food, 

 exhale more carbonic acid and nitrogen in proportion to their 

 weight than the herbivora when living upon their ordinary food. 

 It has long been supposed that the respiration of birds was far 



