644 



CIRCULATION. 



mals, that is to say, the arterial blood returning 

 from the lungs to the left auricle (Jig. 314, h) 

 is directed entirely into the arteries of the 

 system (A) from the left compartment of the 

 ventricle (H), and the venous blood brought 

 back to the right auricle (h 1 ) by the vense 

 cavae (V v*) is directed wholly into 'the pul- 

 monary vessels (P) by the right ventricular 

 compartment ( H'). 



Fig. 316. 



Lacerta ocellata. 



In all Reptiles, however, the descending aorta 

 is formed by the union of two branches, the 

 right and left aortic arches (Jigs. 314, 315, 316, 

 and 317, A', A) ; the right corresponds with the 

 systemic aorta of birds, and rises from the 

 left ventricular compartment, the left arch joins 

 the right on the back, and leads generally 

 from the right ventricular cavity into the 

 descending aorta. The arteries of the head 

 and upper extremities (fg. 314, a*), arising 

 from the right aorta (A 1 ), which corres- 

 ponds with the aorta of birds, and is con- 



nected with the left ventricular compartment, 

 are supplied with highly arterialized blood 

 proceeding directly from the lungs. The left 

 arch of the aorta (A), being connected on 

 the other hand with the right ventricular com- 

 partment (H'), obtains, like the pulmonary 

 artery, venous blood from the right auricle; and 

 consequently the common trunk of the aorta, 

 formed by the union of the right and left aortic 

 arches, must carry to the posterior parts of the 

 body a mixture of arterial and venous blood.* 

 It may be remarked, however, that in the Turtle 

 and some Lizards the left aortic arch does not 

 join the right upon the back until after it (the 

 left) has given off the great creliac or rather 

 visceral artery, which supplies the whole of the 

 alimentary canal and digestive organs with ve- 

 nous blood (Jig. 315, I). The left aorta is thus 

 much diminished in size before it sends its com- 

 paratively small communicating branch to the 

 right.f From this disposition of the parts, it is 

 obvious that in these animals the abdominal 

 viscera must receive the greater part of the 

 venous blood brought from the right side of the 

 heart by the left aortic arch, while the right 

 aortic arch which gives the carotid, brachial, ver- 

 tebral, intercostal, and other arteries must carry 

 to the parts it supplies in the first part of its 

 course nearly pure arterial blood, and, after 

 it is joined by the left, blood which contains 

 a small proportion only of the dark or venous 

 kind. In the Turtle, some Lizards and Ser- 

 pents again, the arterial and venous blood must 

 be mixed in the ventricular cavity though par- 

 tially divided ; the two streams of blood pro- 

 pelled into the aortic and pulmonary vessels 

 must therefore be nearly of the same kind, and 

 thus a part only of the blood which is sent to 

 the lungs is made to undergo a respiratory 

 change. In some of the Chelonia, the exis- 

 tence of ductus arteriosi, leading from the pul- 

 monary artery on each side into the arch of the 

 aorta, insures a still more complete mixture of 

 the arterial and venous blood .J 



In most of the adult Batrachia the ventricle 

 (Jig. 317, H), being single and giving rise to 

 one arterial trunk only (A), the pulmonary 

 arteries (P', P) derive their blood from the 

 great systemic aortic trunk of which they are 

 branches; one coming off from each of the 

 aortic arches which unite to form the descend- 

 ing aorta. The venous blood returning from 

 the system ( V *) to the right auricle, is mixed 

 in the common cavity of the ventricle with the 

 arterial blood returning to the left auricle by 

 the pulmonary veins (/>), and this mixed blood 

 being propelled into the aortic bulb is distri- 

 buted in part to the system and in part to the 

 lungs. In these animals then, only a small 

 quantity of a mixed blood is exposed to the 

 action of the air in the lungs, which, from the 

 simplicity of their structure, offer only a con- 



* In the Crocodile, the left branch coming from 

 the right ventricle is small and very short. 



t See Bojanus' beautiful Anat. Monography of 

 the Tortoise. 



| In this respect, as well as in the mode of 

 origin of the left aortic arch, the Tortoise and Tur- 

 tle differ from one another. 



