804 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



Fig. 314. 



inferior is situated posteriorly, and passes into the auricle in a direction 

 from right to left, transversely to the axis of the heart. A nearly ver- 

 tical curtain or valve at the same time projects behind the orifice of the 

 superior vena cava and in front of the orifice of the inferior. This cur- 

 tain is formed by the lower and right hand edge of the septum of the 

 auricles, which, as above mentioned, is incomplete at this time, and which 

 terminates inferiorly and toward the right in a crescentic border, leaving 

 an oval opening, the foramen ovale. The stream of blood, coming from 

 the superior vena cava, falls in front of this curtain, and passes down- 

 ward, through the auriculo-ventricular orifice, into the right ventricle. 

 But the inferior vena cava, being farther back and directed transversely, 

 opens, properly speaking, not into the right auricle, but into the left ; 

 for its stream of blood, falling behind the curtain above mentioned, 

 passes across, through the foramen ovale, into the cavity of the left auri- 

 cle. This direction of the current 

 of blood, coming from the inferior 

 vena cava, is further secured by a 

 special membranous valve, which 

 exists at this period, termed the 

 Eustachian value. This valve, 

 which is very thin and transparent 

 (Fig. 314, /), is attached to the an- 

 terior border of the orifice of the 

 inferior vena cava, and terminates 

 by a crescentic edge, directed to- 

 ward the left ; thus standing as an 

 incomplete membranous partition 

 between the cavity of the inferior 

 vena cava and that of the right 

 auricle. A bougie, placed in the 

 inferior vena cava, as shown in Fig. 

 314, lies quite behind the Eusta- 

 chian valve, and passes through 

 the foramen ovale, into the left 

 auricle. 



The two streams of blood, there- 

 fore, coming from the superior and 

 inferior venae cavse, cross each other 

 upon entering the heart. This 

 crossing does not take place in the 

 cavity of the right auricle ; but, owing to the position and direction of 

 the two veins, the stream coming from the superior vena cava enters the 

 right auricle, while that from the inferior passes almost directly into 

 the left. 



It also appears, from the relative position of the aorta, pulmonary 

 artery, and ductus arteriosus, at this time, that the arteria innominata, 

 together with the left carotid and left subclavian arteries, are given off 



HEART OF THE HUMAN F<ETITS, at 

 the end of the sixth month ; from a specimen 

 in the author's possession. a. Inferior vena 

 cava. ft. Superior vena cava. c. Cavity of 

 the right auricle, laid open from the front. 

 d. Appendix auricularis. e. Cavity of the 

 right ventricle, also laid open. /. Eustachian 

 valve. The bougie which is placed in the in. 

 ferior vena cava, can be seen passing behind 

 the Eustachian valve, just below the point 

 indicated by /, then crossing behind the 

 cavity of the right auricle, through the 

 foramen ovale, to the left side of the heart. 



