318 



COMPARATIVE ANATOMY 



come into use, but its position can still be recognised as a thin area 

 {fossa otalis) in the atrial septum, surrounded by a fold (annuhs 

 ovalis). Extending from this to the base of the postcaval and right 

 precaval respectively are two folds, known as the Eustachian and 



r«.#..... 



faTJi 



Fig. 260.— Heart of Human Ecetus (8th Month). A, From the right, and B, 

 from the left side. (After Rose.) The walls of the atrium and ventricle are 

 partly removed in each figure. 



Va.s, left sinu-auricular valve, fused with the septum atriorum {S.a,V.a.J); 

 Va.Th, Thebesian valve, in direct connection with the Eustachian valve 

 (Va.E] ; L. V, left atrium ; F.o.v, foramen ovale ; V.c.s, left precaval ; V.c.i, 

 postcaval; A.o, aorta; P, P.d,P.s, pulmonary artery; DB, ductus Botalli 

 (ductus arteriosus) ; L.v. pulmonary vein ; V.c.c, coronary vein. 



Thebesian valves (Fig. 260, a) ; these represent the remains of the 

 right sinu-auricular valve, and serve in the embryo to conduct the 

 blood from the right atrium into the left. 



Great variations are seen in the mode of origin of the carotids 

 and subclavians from the arch of the aorta in Mammals. Thus 



4^^ 



Fig. 261. — Five Different Modes of Origin of the Great Vessels from 

 THE Arch op the Aorta in Mammals. 



Ao, aortic aich , Ih, the, brachiocephalic trunk ; c, carotids ; s, subclavians. 



there may be a IracMocephalic or innorainate irimh on either side 

 (Fig. 261, a) ; or an unpaired common brachiocephalic, from which 

 the carotid and subclavian of one or both sides arise (B, C, E) ; or, 



