348 



ZOOLOGY 



SECT 



the Lacertilia, Ophidia, and Chelonia (Fig. 1013) the structure is 



essentially what has been described in Lacerta, the ventricular 



septum being well-developed, but not completely closing off the 



left-hand portion of the 

 cavity of the ventricle 

 from the right (cavum pul- 

 monale). The left-hand 

 portion, which is much the 

 larger, is further imperfectly 

 divided into two parts the 

 cavum arteriosum on the 

 s\ left and the cavum venosum 

 on the right by the two 

 elongated flaps of the 

 y auriculo-ventricular valve, 

 -As which project freely into 

 the cavity of the ventricle. 

 From the cavum pulmonale 

 arises the pulmonary artery, 

 and from the cavum 

 venosum the right and left 

 aortic arches. When the 

 auricles contract the cavum 

 venosum becomes filled with 

 venous blood from the right 



auricle, the cavum arteriosum with arterial blood from the left 



auricle ; the cavum pulmonale becomes filled with venous blood 



which flows into it past the edges 



of the incomplete septum. When 



the ventricle contracts, its walls 



come in contact with the edge of 



the septum, and the cavum pul- 

 monale is thus cut off from the 



rest of the ventricle. The further 



contraction consequently results in 



the venous blood of the cavum 



pulmonale being driven out through 



the pulmonary artery to the lungs, 



while the blood which remains in 



the ventricle (arterial and mixed) 



is compelled to pass out through the 



aorta. But in the Crocodilia (Fig. 



1014) the cavity is completely 



divided, so that there we may speak 



of distinct right and left ventricles. 



From the right arises the pulmonary artery and the left aortic 



arch : from the left the right aortic arch only. The right and left 



FIG. 1012. Heart of Lacerta muralis, ventral 

 view. A, A. auricles ; Ap. pulmonary artery ; 

 As, As*, subclavian arteries ; Ci, post-caval ; J. 

 jugular vein ; Ra, aortic arches (made up on 

 either side of two embryonic arches, 1 and 2) ; 

 tr, aortic root ; V. ventricle ; Vp, pulmonary 

 vein ; Vs, subclavian vein. (From Wiedersheim's 

 Comparative Anatomy.) 



Ao 



cavum pulmonale ; C.v. cavum veno- 

 sum ; L. A. left auricle ; L. Ao. left 

 aortic arch ; P. A. pulmonary artery ; 

 R. A. right auricle ; R. Ao. right aortic 

 arch ; s, arrow showing the course of 

 blood in left aorta ; t, in right aorta ; 

 v., v'. auriculo-ventricular valves ; w, 

 arrow showing the course of blood in 

 left auriculo-ventricular aperture ; x, in 

 right ; y, between cavum venosum and 

 cavum pulmonale ; z, in pulmonary 

 artery. (After Huxley.) 



