DEVELOPMENT OF THE VEINS. 



927 



importance, is more nearly retained, but indications of a transference of blood from the intracranial 

 portions of the internal jugular system to the external vessel are to be.seen in the emissary veins. 

 At first each internal jugular opens independently into the right auricle through the 

 corresponding duct of Cuvier (Fig. 777, A), but later a communicating branch extending 

 obliquely across from the left to the right vein is developed (Fig. 777, B), and thereafter the 



Fig. 777. 



Anterior cardinal 



(internal jugular) 



Subclavian 



Cuvierian duct y— ^ 



Primary inf. cava 

 Posterior cardinal 



— T Internal jugular 

 |-^ External jugular 

 Subclavian 



Left innominate 



Oblique vein of 



left auricU 

 Coronary sinus 



Inferior vena cava 



Diagrams illustrating development of superior vena cava; ^.primary symmetrical arrangement ; .5, establishment 

 of transverse connection ; C, atrophy on left side and persistence on right side of superior vena cava. 



lower portion of the left vein degenerates until it is represented only by the small oblique vein of 

 the left auricle, opening into the coronary sinus, which is the persisting left ductus Cuvieri 

 (Fig. 777, C). The oblique connecting branch becomes the left innominate vein of adult 

 anatomy, and the portion of the right anterior cardinal below the point where it is joined by the 

 innominate, together with the right ductus Cuvieri, becomes the superior vena cava. 



"The Inferior Vena Cava. — The posterior cardinals persist in part as the azygos veins, but 

 their history is so intimately associated with that of the inferior vena cava that an account of tlie 

 development of the latter may first be presented. In the early stages of development the only 

 portion of the inferior vena cava which exists is the portion which intervenes between the entrance 

 of the hepatic veins and the right auricle, this portion representing the terminal part of the ductus 



Internal jugular 

 (ant. cardinal 



Duct of Cuvier 



Posterior cardinal 

 Inf. vena cava 



Renal 

 Subcardinal 



Azygos 



Renal 



Coronary sinus 



Hemiazygos 



Inferior vena cava 



Iliac 



Diagrams illustrating developmental changes leading to formation of inferior caval and azygos veins. 



venosus (page 705). Branches which pass to the posterior cardinal veins from the mesentery 

 anastomose longitudinally to form on each side of the body a venous stem which has a course 

 parallel to that of the cardinals, with which it unites below ( Fig. 77S, A). This is the subcarditial 

 vein, the two vessels of opposite sides of the body being united with one another and with the 

 cardinals by a strong cross-branch which joins the cardinals opposite the point of entrance into 



