580 EMBRYOLOGY. 



There are in it separate openings for the two Cuvierian ducts the 

 future venae cavae superiores and an opening distinct from them for 

 the veins coming from the viscera below (the future cava inferior). 



The metamorphoses in the region of the Cuvierian ducts begin 

 with a change in their position. Their course from above down- 

 ward becomes more direct. At the same time, like the sinus 

 venosus, they emerge from the niveau of the transverse septum and 

 lateral walls of the trunk into the body-cavity and carry before them 

 the serous membrane, with which they are covered, as a crescent- 

 shaped fold, which contributes to the formation of the pericardial 

 sac, and has been already described as the j)leuro-j)ericardial fold. 

 By fusing with the mediastinum the Cuvierian ducts pass from the 

 walls of the trunk into the latter and come to lie nearer together in 

 the median plane. Of their affluents the jugular veins gradually 

 predominate over the cardinal veins (fig. 322 B). There are three 

 reasons for this. First, the anterior part of the body, and especially 

 the brain, far outstrips in growth the posterior part ; secondly, there 

 arises in this region a competitor of the cardinal veins, the inferior 

 vena cava, which assumes in place of them the function of returning 

 the blood. Thirdly, when the anterior limbs are established, the 

 venae subclavise (s) empty into the jugulares. Consequently the 

 lower portion of the jugular, from the entrance of the subclavia 

 onward, now appears as the immediate continuation of the Cuvierian 

 duct, and together with it is designated as superior vena cava 

 (fig. 322 B csd). 



There exists between the right and left sides a difference in the 

 course of the superior venae cavae, which, as GEGENBAUR has pointed 

 out, is the cause of the asymmetry that is developed in Man. 

 While the right vena cava superior (fig. 322 B csd) descends more 

 directly to the heart, the left (ess) describes a somewhat longer 

 course. Its terminal portion is bent from the right to the left 

 around the posterior [dorsal] wall of the atrium, where it is imbedded 

 in the coronal furrow and receives the blood from the coronal vein 

 (cc) of the heart. 



In Reptiles, Birds, and many Mammals a stage of this kind, with 

 two venae cavae superiores, becomes permanent ; in Man it exists 

 only during the first months. Then there is a partial degeneration 

 of the left vena cava superior. The degeneration is initiated by the 

 formation of a transverse anastomosis (fig. 322 B as) between the 

 right and left trunks. This conveys the blood from the left to 

 the right side, where the conditions are more favorable for the 



