CHEMICAL EFFECTS OF RESPIRATION. 339 



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 ana- 

 logous to those artificial operations which effect 

 the combustion of charcoal. The food supplies 

 the fuel, which is prepared for use by the di- 

 gestive organs, and conveyed by the pulmonary 

 arteries to the place where it is to undergo com- 

 bustion : the diaphragm 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. 



It becomes an interesting problem to deter- 

 mine whether this analogy may not be further 

 extended ; and whether the combustion of car- 

 bon, which takes place in respiration, be not the 

 exclusive source of the increased temperature, 

 which all animals, but more especially those 

 designated as tvann-blooded, usually maintain 

 above the surrounding medium. The uniform 

 and exact relation which may be observed to 

 take place between the temperature of animals 

 and the energy of the respiratory function, or 

 rather the amount of the chemical changes 

 induced by that function, affords very strong 

 evidence in favour of this hypothesis. The 

 coincidence, indeed, is so strong, that notwith- 

 standing the objections that have been raised 



* See the note at page 334. 



