334 THE VITAL FUNCTIONS. 



content myself, therefore, with a concise state- 

 ment of such of the leading facts relating to this 

 function, as have now, by the labours of modern 

 physiologists, been satisfactorily established, and 

 which serve to elucidate the beneficent intentions 

 of nature in the economy of the animal system. 



Atmospheric air acts without difficulty upon 

 the blood, while it is circulating through the 

 vessels which are ramified over the membranes 

 lining the air cells of the lungs ; for neither 

 these membranes, nor the thin coats of the vessels 

 themselves, present any obstacle to the trans- 

 mission of chemical elements from the one to the 

 other. The blood being a highly compound 

 fluid, it is exceedingly difficult to obtain an ac- 

 curate analysis of it, and still more to ascertain 

 with precision the different modifications which 

 occur in its chemical condition at different times : 

 on this account, it is scarcely possible to deter- 

 mine, by direct observation, what are the exact 

 chemical changes, which that fluid undergoes 

 during its passage through the lungs ; and we 

 have only collateral evidence to guide us in the 

 inquiry.* 



* Some experiments very recently made by Messrs. Macaire 

 and Marcet, on the ultimate analysis of arterial and venous 

 blood, taken from a rabbit, and dried, have shown that the 

 former contains a larger proportion of oxygen than the latter ; 

 and that the latter contains a larger proportion of carbon than 

 the former : the proportions of nitrogen and hydrogen being the 



