DEVELOPMENT OF THE HEART. 



681 



Fig. 271. 



the left auricle. This direction of the current of blood, coming 

 from the inferior vena cava, is further secured by a peculiar mem- 

 branous valve, which exists at this period, termed the Eustachian 

 valve. This valve, which is very thin and transparent (Fig. 271, /), 

 is attached to the anterior border of the orifice of the inferior vena 

 cava, and terminates by a crescentic edge, directed toward the left ; 

 the valve, in this way, standing 

 as an incomplete membranous 

 partition between the cavity of 

 the inferior vena cava and that 

 of the right auricle. A bougie, 

 accordingly, placed in the in- 

 ferior vena cava, as shown in 

 Fig. 271, lies naturally quite 

 behind the Eustachian valve, 

 and passes directly through 

 the foramen ovale, into the left 

 auricle. 



The two streams of blood, 

 therefore, coming from the su- 

 perior and inferior vena3 cavoa, 

 cross each other upon entering 

 the heart. This crossing of the 

 streams does not take place, 

 however, as it is sometimes 

 described, in the cavity of the 

 right auricle ; but, owing to the 

 peculiar position and direction 

 of the two veins at this period, 

 with regard to the septum of 



the auricles, the stream coming from the superior vena cava enters 

 the right auricle exclusively, while that from the inferior passes 

 almost directly into the left auricle. 



It will also be seen, by examining the positions of the aorta, pul- 

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

 innominata, together with the left carotid and left subclavian, are 

 given off from the arch of the aorta, before its junction with the 

 ductus arteriosus, and this arrangement causes the blood of the two 

 venae cavse, not only to enter the heart in different directions, but 

 also to be distributed, after leaving the ventricles, to different parts 

 of the body. (Fig. 272.) For the blood of the superior vena cava 



HKARTOF HUMAN fee. rrs, at the end of the 

 sixth month ; from a specimen in the author's pos- 

 session. a. Inferior vena cava. b. Superior vena 

 cava c. Cavity of riirlit auricle, laid open from 

 the front, d. Appendix auricularis. e. Cavity of 

 rik'ht ventricle, also laid open. /. Eustachian valve. 

 The bougie, which is placed in the inferior vena 

 cava, can be seen passing behind the Eustachian 

 valve, just below the point indicated by /, then 

 crossing behind the cavity of the right au icle, and 

 passing through the foramen ovale, to the left side 

 of the heart. 



