DEVELOPMENT OF THE YEIXS. 



485 



in the dorsal region between the posterior vertebral veins a communicating branch 

 passes obliquely across the middle line from right to left. The communicating branch 

 between the primitive jugular veins forms the left innominate vein. The portion of 

 vessel between the termination of the right subclavian vein and the termination of the 

 communicating branch becomes the right innominate vein. The portion of the primi- 

 tive jugular vein below the communicating vein, together with the right duct of Cuvier, 

 forms the vena cava superior, while the cardinal vein opening into it is the extremity 

 of the great vena azygos. On the left side, the portion of the primitive jugular vein 

 placed below the communicating branch, and the cardinal and posterior vertebral 

 veins, together with the cross branch between the two posterior vertebral veins, are 



Fig. 330. 



Fig. 330, A. and B. DIAGRAMMATIC OUTLINES OP THE VESTIGE OP THE LEPT SUPERIOR 

 CAVA AND OF A CASE OP ITS PERSISTENCE (sketched after Marshall). 



A, Brachio-cephalic veins with the superior intercostal, azygos, and principal cardiac 

 veins. 



B, the same in a case of persistence of the left superior cava, showing its communica- 

 tion with the sinus of the coronary vein. The views are supposed to be from before, the 

 parts of the heart being removed. 



1, 1', the internal jugular veins ; 2,2', subclavian veins ; 3, right innominate ; 3', right 

 or regular superior cava ; 4, in A, the left innominate ; in B, the transverse or communi- 

 cating vein between the right and left superior venas cavae ; 5, in A, the opening of the 

 superior intercostal vein into the innominate ; 5', vestige of the left superior cava or duct 

 of Cuvier ; 5, 5', in B, the left vena cava superior abnormally persistent along with a 

 contracted condition of 4, the communicating vein ; 6, the sinus of the coronary vein ; 6', 

 branches of the coronary veins ; 7, the superior intercostal trunk of the left side, or left 

 cardinal vein ; 8, the principal azygos or right cardinal vein ; 7', 8', some of the upper 

 intercostal veins ; 9, the opening of the inferior vena cava, with the Eustachian valve. 



converted into the left superior intercostal and left superior and inferior azygos veins. 

 The variability in the adult arrangement of these vessels depends on the various 

 extent to which the originally continuous vessels are developed or atrophied at one 

 point or another. The left duct of Cuvier is obliterated, except at its lower end, 

 which always remains pervious as the coronary sinus. Even in the adult, traces of 

 the existence of this vessel can always be recognised in the form of a fibrous band, 

 or sometimes even a narrow vein, which descends obliquely on the left auricle ; and 

 in front of the root of the left lung there remains a small fold of the serous membrane 



