30 i THE VriAL FUNCTIONS. 



established b}^ the researches of Spallanzani, and 

 still more by those of Humboldt and Provencal, on 

 fishes, that nitrogen is actually absorbed. A con- 

 firmation of this result has recentlv been obtained 

 by Messrs. Macaire and Marcet, who have found 

 that the blood contains a larger proportion of 

 nitrogen than the chyle, from which it is formed. 

 We can discover no other source from Avhich chyle 

 could acquire this additional quantity of nitrogen, 

 during its conversion into blood, than the air of the 

 atmosphere, to which it is exposed in its passage 

 through the pulmonary vessels.* 



According to these views of the chemical objects 

 of respiration, the process itself is analogous to 

 those artificial operations which effect the com- 

 bustion of charcoal. The food supplies the fuel, 

 w hich is prepared for use by the digestive organs, 

 and conveyed by the pulmonary arteries to the 

 place where it is to undergo combustion : the dia- 

 phragm is the' bellows, which feeds the furnace 

 with air ; and the trachea is the chimney, through 

 which the carbonic acid, which is the product of 

 the combustion, escapes.t 



It becomes an interesting problem to determine 

 whether this analogy may not be farther extended ; 

 and whether the combustion of carbon, which takes 

 place in respiration, be not the exclusive source of 



* See the note at page 300. 



t It is now generally believed that the combination of the 

 absorbed oxygen with the carbon contained in the blood takes 

 place, not so much in the vessels of the lungs, as in the course of 

 the systemic circulation. It has been found that carbonic acid 

 exists in larger proportion in venous than in arterial blood : a fact 

 which is decidedly in accordance with this view of the process. 



