97 



of the body. The greater perfection of the orga- 

 nization of these tribes seems to have required 

 that the blood should be in general, more deci- 

 dedly of two kinds arterial and venous than 

 was necessary in those of a lower grade ; while, 

 at the same time, their amphibious habits did not 

 allow of the distinctly double heart which is pro- 

 per to birds and mammiferous animals, the re- 

 spiration of which is not liable to be in the same 

 way impeded. 



In birds, for the first time, we meet with a 

 heart double, as well in structure, as in function ; 

 the right compartment of it being constantly and 

 exclusively pulmonary, and the left equally con- 

 stantly and exclusively systematic ; and though 

 these two compartments are united into one 

 heart, and placed on one side of the body, they 

 would have performed their distinct offices per- 

 haps equally well, had they been quite separate 

 from each other, and placed one on one side of 

 the body, and the other on the other. The right 

 cavity of the heart of birds gets its blood, like the 

 single heart of fishes, from the large veins of the 

 body, and transmits it through the respiratory 

 organ ; whence, on its return, it passes, not, as 

 in fishes, directly into the large artery in the 

 course of the spine, but first into the left cavity 

 of the heart, by which it is propelled, as by the 

 heart of the frog, into the artery in question, to 

 be at length returned to the right cavity by the 

 large veins. In this course the blood is directed 

 as it is also through the heart and large vessels 



