^52 



I shall only add, that there is no difficulty in suggesting 

 probable sources from which nitrogen might be derived for 

 combination with unazotized alimentary principles ; and that, 

 from all these considerations, there seems sufficient reason 

 to believe, contrary to the opinion of Liebig, that the 

 digestive and assimilative functions of herbivorous animals 

 do possess a sufficient converting power to form muscular 

 tissue, or other tissues composed of azotized principles, from 

 non-azotized alimentary substances. 



One important office is attributed by Liebig to non- 

 azotized substances taken as food by animals, which was 

 before alluded to, and which must now be briefly explained. 

 This is its subserviency to the maintenance of the elevated 

 temperature of the animal body, through the medium of 

 the respiratory function. 



This function consists essentially in the inhalation of 

 oxygen gas into the lungs, from whence it is absorbed into 

 the blood, where it enters into a chemical combination 

 with portions of the carbon and hydrogen of some of the 

 principles it there meets with, and is again discharged from 

 the lungs as carbonic acid gas and watery vapour. 



Whenever this combination of a combustible with oxygen 

 is effected, it is accompanied with a development of caloric, 

 more or less, according as the combination is more or less 

 rapid and extensive ; it is, in fact, combustion^ and is of the 

 same essential nature whether it take place with rapidity, 

 as in the burning of fuel in our fire-places, or of gas in our 

 lamps ; or more slowly, as in a damp hay-stack, or by the 

 process of animal respiration. To this action, constantly 

 going on during the life of the animal, physiologists now 

 attribute that elevated temperature which its body maintains 

 during the same period. 



The sources of the carbon and hydrogen employed in this 

 function may be various ; when food has been fully and 



