DEVELOPMENT OF THE VASCULAR SYSTEM. 699 



right to left. A nearly vertical curtain or valve projects at the same 

 time behind the orifice of the superior vena cava and in front of the 

 orifice of the inferior. This curtain is formed by the incomplete septum 

 of the auricles, which terminates inferiorly and toward the right in a 

 crescentic border, at the foramen ovale. The stream of blood, coming 

 from the superior vena cava, falls in front of this curtain, and p; 

 downward, through the auriculo-ventricular orifice, into the right ven- 

 tricle. But the inferior vena cava, owing to its posterior and trans- 

 verse position, opens, properly speaking, not into the right auricle, 

 but into the left. Its stream of blood, falling behind the above-men- 

 tioned curtain, passes across, through the foramen ovale, into the left 

 auricle. This direction of the cur- 

 rent from the inferior vena cava 

 is further secured by a second 

 membranous partition, which ex- 

 ists at this period, termed the 

 Eustachian valve. This valve, 

 which is very thin and trans- 

 parent (Fig. 250, /), is attached 

 in front of the orifice of the in- 

 ferior vena cava, and 



FIG. 250. 



by a crescentic edge toward the 

 left; thus standing between the 

 cavities of the inferior vena cava 

 and right auricle. A bougie, placed 

 in the inferior vena cava, as in 

 Fig. 250, lies behind the Eusta- 

 chian Valve, and passes through HEART OFJHE HUMAN FCETUS. at the end of 



the foramen ovale, into the left 

 auricle. 



The two streams of blood, there- 

 fore, coming from the superior and 

 inferior venae cavae, cross each 

 other on entering the heart. 

 Owing to the position of the two 

 veins and their adjacent valves, the stream coming from the superior 

 vena cava enters the right auricle, while that from the inferior passes 

 transversely into the left. 



The relations of the aorta, pulmonary artery, and ductus arteriosus 

 at this time are such that the arteria innominate, the left carotid and 

 left subclavian arteries are given off from the arch of the aorta, before 

 the junction of the ductus arteriosus ; and thus the blood-currents of 

 the two venae cava? are distributed, after leaving the ventricles, to 

 different parts of the body (Fig. 251). The blood of the superior 

 vena cava passes through the right auricle into the right ventricle, 

 thence through the pulmonary artery and ductus arteriosus, to the 



the sixth month. a. Inferior vena cava. 

 b. Superior vena cava. <. Cavity of the right 

 auricle, laid open from the front, d. Appendix 

 auricularis. e. Cavity .f tin- ri-ht ventricle. 

 /. Eustaehian valve. The bougie, placed in 

 the inferior vena cava, can be seen passing 

 behind the Eustachiau valve, just below the 

 point/, then crossing, behind the right auricle, 

 through the foramen ovale, to the left side of 

 the heart. 



