CIRCULATORY ORGANS 



321 



trance of the veins into the atrium show that it has not entirely disappeared. 

 The atrial septum is complete and is continued forward into the ventricle, par- 

 tially {Sphenodon, turtles, squamata) or completely (crocodiles, fig. 342, B) 

 separating it into arterial and venous halves. The arterial trunk often shows 

 a spiral twist (fig. 346). 



The eculiar relations of the aortic arches have already been mentioned 

 (p. 301 ; fig. 319, £, F), but it should be mentioned that there is often a connexion 

 between the right and left fourth aortic arches — the foramen of Panniza (fig. 

 342, B), so that blood can pass from one side to the other. It is stated that with 

 quiet breathing blood goes from the right arch into the left; with rapid respira- 

 tion the flow is in the other direction. But under ordinary conditions the blood 

 in the right arch is predominantly arterial, that in the left largely venous. As 



Fig. 342. — A, dorsal view of heart of crocodile, after Rdse; 5, ventral scheme of 

 heart laid open, after Greil. b, remnants of Botall's duct; c, carotid trunk; cv, cardinal 

 vein; ca, ccEliac artery; d, dorsal aorta; //>, foramen Panizzae; k, coronary arteries; j, 

 jugular vein; /, left aortic arch; la, left atrium; Iv, left ventricle;'^a, pulmonary artery; 

 PCy postcava; pv, pulmonary vein; r, right aortic (fourth) arch; ra, right atrium; ro, 

 right ventricle; si, sr, left and right subclavian arteries; v, ventricle. 



the latter is the chief supply of the alimentary canal (fig. 342, ca) this is not 

 physiologically as anomalous as it would at first sight appear. Correlated with 

 these differences of the fourth arch on the two sides is the fact that both of the 

 subclavian arteries arise from the left arch, while usually these are united with 

 the carotids, so that subclavians and carotids form for a distance a common 

 trunk, the brachiocephalic artery. In the snakes the subclavians are lost, 

 while in the same group and in the snake-like lizards one of the pulmonary 

 arteries disappears along with the degeneration of the corresponding lung. 



In many reptiles the anterior parts of the postcardinal veins are replaced by 

 the vertebral veins and the blood from the excretory organs is wholly returned to 

 the heart by the postcava. That from the posterior parts of the body can re- 

 turn either by the renal portal system (fig. 347) or by the paired abdominal 



