CHEMICAL EFFECTS OF RESPIRATION. 237 



is only of late years that we may be said to have obtained 

 any accurate knowledge as to the real nature of this impor- 

 tant function; and there is perhaps no branch of physiology 

 which exhibits in its history a more humiliating picture of 

 the wide sea of error in which the human intellect is prone 

 to lose itself, when the path of philosophical induction is 

 abandoned, than the multitude of wild and visionary hypo- 

 theses, devoid of all solid foundation, and perplexed by the 

 most inconsistent reasonings, which formerly prevailed with 

 regard to the objects and the processes of respiration. To 

 give an account, or even a brief enumeration of these theo- 

 ries, now sufficiently exploded, would be incompatible with 

 the purpose to which I must 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 have 

 now, by the labours of modern physiologists, been satisfac- 

 torily established, and which serve to elucidate the benefi- 

 cent intentions of nature in the economy of the animal sys- 

 tem. 



Atmospheric air acts without difficulty upon the blood 

 while it is circulating through the vessels which are rami- 

 fied over the membranes lining the air cells of the lungs; 

 for neither these membranes, nor the thin coats of the ves- 

 sels themselves, present any obstacle to the transmission of 

 chemical elements from the one to the other. The blood 

 being a highly compound fluid, it is exceedingly difficult to 

 obtain an accurate 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 determine, by direct observation, what 

 are the exact chemical changes, which that fluid undergoes 



* For an account of the history 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 which has 

 yet appeared. 



