.'300 THE VITAL FUNCTIONS. 



confine myself in this treatise.* I shall content 

 myself, therefore, with a concise statement of such 

 of the leading facts relating to this function, as 

 liave 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 transmission of chemical ele- 

 ments from the one to the other. The blood beino; 

 a highly compound fluid, it is exceedingly difficult 

 to obtain an accurate analysis of it, and still more 

 to ascertain with precision the different modifica- 

 tions which occur in its chemical condition at dif- 

 ferent times : on this account, it is scarcely possible 

 to determine, 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^t 

 The most obvious effect resulting from the action 



of the air is a change of colour from the dark purple 



* For an account of the liistory of the various chemical theories 

 which have prevailed on this interesting' department of Physiology, I 

 must refer to the " Essay on Respiration," by Dr. Bostock, and 

 also to the " Elementary System of Physiology," by the same 

 author, which latter work comprises the most comprehensive and 

 accurate compendium of the science that has yet appeared. 



t 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 



