OBJECTS OF CIRCULATION. 205 



which it is their peculiar office to prepare. A 

 second essential object of the circulation, is to 

 transmit the nutritive juices to particular organs, 

 where it may be purified by the separation of cer- 

 tain redundant elements, the presence or accumu- 

 lation of which would impair its salutary proper- 

 ties, and even prove destructive to life. These ele- 

 ments are Carbon, Hydrogen, and Nitrogen. The 

 elimination of the two last is effected more espe- 

 cially by the liver and the kidneys : the separa- 

 tion of the Carbon by its combination with Oxygen 

 is the office of the respiratory organs, where the 

 blood is subjected to the influence of atmospheric 

 air ; a process, which in all warm-blooded animals, 

 combined with the rapid and extensive distribution 

 of the blood, diffuses and maintains throughout the 

 system the high temperature required by the 

 greater energy of their functions. Hence it neces- 

 sarily follows that the particular mode in which 

 the circulation is conducted in each respective 

 tribe of animals, must influence every other func- 

 tion of their economy, and must, therefore, consti- 

 tute an essential element in determining their phy- 

 siological condition. We find, accordingly, that 

 among the characters on which systematic zoolo- 

 gists have founded their great divisions of the ani- 

 mal kingdom, very great importance is attached to 

 those derived from differences of structure in the 

 organs of circulation. 



A comprehensive survey of the different classes 

 of animals with reference to this function, enables 

 us to discern the existence of a gradation of organs, 

 increasing in complexity as we ascend from the 

 lower to the higher orders ; and showing that here, 



