CHEMICAL EFFECTS OF RESPIRATION. 241 



gestive organs, and conveyed by the pulmonary arteries to 

 the place where it is to undergo combustion: the diaphragm 

 is the bellows, which feeds the. furnace with air; and the tra- 

 chea is the chimney, through which the carbonic acid, which 

 is the product of the combustion, escapes. 



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 the increased temperature, which 

 all animals, but more especially those designated as warm- 

 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 ener- 

 gy 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 coinci- 

 dence, indeed, is so strong, that, notwithstanding the objec- 

 tions that have been raised against the theory founded upon 

 this hypothesis, from some apparent anomalies which occa- 

 sionally present themselves, we must, I think, admit that it 

 affords the best explanation of the phenomena of any theory 

 yet proposed, and that, therefore, it is probably the true one. 



The maintenance of a very elevated temperature appears 

 to require the concurrence of two conditions; namely, first, 

 that the whole of the blood should be subjected to the influ- 

 ence of the air, and, secondly, that the air should be pre- 

 sented to it in a gaseous state. These, then, are the circum- 

 stances which establish the great distinction between warm 



O 



and cold-blooded animals; a distinction which at once stamps 

 the character of their whole constitution. It is the condition 

 of a high temperature in the blood which raises the quadru- 

 ped and the bird to a rank, in the scale of vitality, so far 

 above that of the reptile: it is this which places an insupera- 

 ble boundary between mammalia and fishes. However the 

 warm-blooded Cetacea, who spend their lives in the ocean, 

 may be found to approximate in their outward form, and in 

 their external instruments of motion, to the other inhabitants 

 VOL II. 31 



