412 COMPARATIVE ANATOMY 



are also present at the origins of the pulmonary artery and aorta 

 both in Birds and Mammals. 



As regards the origin of the great vessels, Birds are distinguished 

 from Mammals by the fact that in them the right, while in Mammals 

 the left, aortic arch persists (Fig. 301, E, F) ; the corresponding 

 arch of the other side in both cases takes part in forming the 

 subclavian artery. Thus both in Birds and Mammals there is only a 

 single aortic arch. As in Amphibians, the posterior arterial arch 

 gives rise to the pulmonary artery. 



The pulmonary veins, two from each lung, open close together 

 into the left atrium (Fig. 317). 



Amongst the more important points in the development of the 



FIG. 316. HEART OF GOOSE (Anser vidgaris), DISSECTED FROM THE RK;HT SIDE. 



(After Rose.) 



The right atrium and ventricle are cut 4 open, and their walls reflected. Ao, 

 aorta ; L. Vi, limbus fossae ovalis ( Vieussenii) a ridge arising from the ventral 

 wall of the right atrium ; the space between this and the septum atriorum is 

 known as the spatium intersepto-valvulare (cf. Figs. 311 and 312). MK, 

 MK 1 , muscular right atrio- ventricular valve ; S.a, septum atriorum ; V.a,d, 

 V.a.s, the two sinu-auricular valves, situated at the entrance of the postcaval ; 

 V.c.c, aperture of coronary vein ; V.C.s.d, right precaval. 



heart may be mentioned the fact that in the embryo the two atria 

 communicate with one another by means of an aperture, the 

 foramen ovale, or by several apertures, formed secondarily in the 

 atrial septum, through which the venous blood from the right 

 atrium passes directly into the left atrium (Fig. 318). This 

 foramen becomes closed later by secondary growths, which are less 

 complicated in Birds and Monotremes than in Placental Mammals, 

 in which a secondary circular septum is found, and when the lungs 



