356 PHILOSOPHY OF ZOOLOGY 



it amounts nearly to half an ounce every hour. Experi- 

 ments are still wanting to determine the relation of the quan- 

 tity to the bulk of the animal, or the amount of the food 

 which is consumed. 



It now remains that we trace the progress of the aerated 

 blood in the Systemic vessels to its final destination. The 

 radicles of the systemic vessels take their rise in the aerat- 

 ing organs, and receive from the terminating twigs of the 

 pulmonic vessels, the blood which has parted with its carbon, 

 and suffered the changes consequent upon the separation of 

 the superfluous portion of that ingredient. These radicles 

 unite into branches, and either transmit the blood directly 

 to the different parts of the body, or suffer muscular enlarge- 

 ments, forming a systemic heart. In quadrupeds, the aerated 

 blood is collected from the lungs by the radicles of the syste- 

 mic veins : these, by their union, form four trunks, which 

 proceed to their common sinus, and, through it, pour their 

 contents into the systemic or left auricle. The blood now 

 enters the systemic ventricle, and, by means of its contrac- 

 tions, is sent into the systemic artery or Aorta, of which it 

 may be considered as an expansion. Through the subdi- 

 visions of the aorta, the blood is conveyed to all the parts 

 of the body. 



The structure of the systemic heart is similar to that of 

 the pulmonic, with this difference, that its walls are much 

 stronger, and, consequently, are fitted for exerting the 

 requisite force in order to propel the blood through the 

 arteries to the remotest parts of the system *. 



* It appears probable, that the arteries themselves, by the contraction 

 of their coats, serve to promote the circulation of the blood, and that these 

 contractions depend on the nerves with which they are supplied. See a 

 paper by Sir E. HOME, On the Influence of the Nerves upon the Action of 

 the Arteries." Phil. Trans. 1814, p. 583. 



