OP NORTH AMERICA, Sgl 



nic or left aorfa. At tijis place, the partition between the two 

 ventricles is not as thick as helow, i)iit tliere is a cartila2;inous 

 septum, whicli is so placed as to tlividc and give tiasticity to 

 the orilice of the pulmonary artery, and the sjjlanchnic or 

 left aorta, I have endeavoured in vain to lind here a direct 

 communication with the left ventricle. Tin pulmonary veins 

 pass under the bronchia, and pierce the pericardium l)ehind; 

 they then unite, and enter the left auricle, which is placed on 

 the upper and posterior part of the base of the heart. The 

 left ventiicle is situated partly behind the other ; it extends 

 a little higher, and seems to t)e rather smaller ; it is furnish- 

 ed, like the other, with two valves, placed l)efore the orilice 

 of the auricle ; it has also an opening into two arterial tubes ; 

 the fnst leads into the left or splanchnic aorta, and is onlv 

 separated from the orifice of the right ventricle into the 

 same artery by the cartilaginous partition : it is bordered by 

 a valve, which nearly closes it, so that a very small quantity 

 of blood only is allowed to pass from this ventricle through 

 the opening. The other |)assage for the blood is placed above; 

 this arterial tul)e divides into //iree branches — one which forms 

 the right or systemic aorta. anotl)cr forming the right sul)cla- 

 vian, and a third the carotid artery, which at first inclines on 

 the left side, s(M»d3 a branch which is the left subclavian, and 

 previous to its enteringthecranium,divides into two branches. 

 This common tube, together with the left aorta, and the 

 canal for the pulmonary arteries, is united, and forms a 

 large arterial sack, siiuatcd al)ove the heart, which is inclosed 

 within the pericaidium : tlie arteries divide into branches 

 as they pierce tlirougli this sack. These three canals, above 

 the ventricles, and l)eforc leaving the pericardium, are ex- 

 tremely enlarged, and form wide baijs, capable of containing 

 more blood than all the cavities of the heart together. Such 

 lalwavs found the structure of the heart in the alligator. 



Thus the iilood coming from tiie body is introduce! into 

 the right auricle ; from this cell it (Inws into the right ven- 

 tricle, which propels it into the pulmonary artery, and partly 



VOL. II. G 2 



