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, 1s 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. 
