180 



Heart. 



Eustachii 



ct.urt. 



Venlr. sin. 



522 a. 1). Heart of an Embryo of Six Months, 



natural size, the auricles laid open. 



The right auricle, Auricula dextra, occupies the right and anterior 

 portion of the base of the organ. Its right or outer wall is the smallest, 

 the left wall is formed by the Septum aurlcidarum. At the posterior half 

 of the septum is the Fossa ovalis, whose floor is thin and translucent ; 

 it is bounded above and at the sides by a prominent border, deficient 

 below, the Annulus ovalis s. Lintlus foraminis ovalis s. Isthmus Vieussenii 

 (see Fig. 521). 



In the heart of the embryo, the Fossa ovalis is a foramen, Foramen 

 ovale ; the blood flowing from the inferior Vc.na cava into the right auricle, 

 mostly arterial, is guided by a semilunar projection, the Eustachian 

 valve, to the Foramen ovalc, through which the greater part passes to 

 the left auricle. 



The inferior Vena cava opens at the posterior wall of the right 

 auricle, the superior Vena cava at the upper wall. The right Appendix 

 auriculae projects forwards and to the left side, overlapping the root of 

 the aorta. The under part of the right auricle is occupied by the 

 auricular-ventricular orifice, which leads into the right ventricle. 



