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THE VASCULAR SYSTEMS 



The fossa ovalis is an oval depression corresponding to the situation of the 

 foramen ovale in the fetus. It is situated at the lower part of the auricular septum, 

 above and to the left of the orifice of the inferior vena cava. In fetal life an 

 opening, the foramen ovale, exists at this point between the two auricles; almost 

 immediately after birth the valve-like edge is pressed down by the increased 

 pressure in the left auricle, and by the tenth day it passes to the annulus and 

 closes the opening. 



The annulus ovalis (limbus fossae ovalis [Vieusseiiii]} is the prominent oval 

 margin of the fossa ovalis. It is most distinct above and at the sides; below, it is 

 deficient. A small, slit-like, valvular opening is occasionally found, at the upper 

 anterior margin of the fossa ovalis, which leads upward beneath the annulus into 

 the left auricle; it is the remains of the fetal aperture between the two auricles. 



FIG. 418. Heart opened to show the interior of the rierht auride and of the two ventricles, from in front. 

 The wall of the right auricle is turned back to show the musculi pectinati and the crista terminalis. The ven- 

 tricular walls and the ventricular septum have been cut. The aortic valve is made more prominently visible in 

 the drawing than it really is in nature. 



The tubercle of Lower (tuberculum intervenosum [Loweri]) is a small projection 

 on the interauricular septum between the fossa ovalis and the opening of the 

 superior vena cava. It is most distinct in the hearts of quadrupeds; in man it is 

 scarcely visible. It was supposed by Lower to direct the blood from the superior 

 vena cava toward the auriculoventricular opening. 



The internal surface of the right auricle is smooth, except in the appendix and 

 adjacent part of the anterior wall of the sinus venosus, where the muscular wall is 

 thrown into parallel ridges resembling the teeth of a comb, and hence named the 



