THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



225 



undergoes regressive changes, its function being taken by a new vessel 

 the inferior vena cava. 



Not long after the appearance of the posterior cardinals, another pair of 

 longitudinal veins appears in the medial part of the mesonephroi. They 

 increase in size as the mesonephroi increase and receive blood from the 

 latter. They also communicate with the cardinals by means of transverse 

 channels which, however, are later broken up as the mesonephroi become 

 more complicated in structure. These vessels are known as the subcardinal 

 veins, or revehent veins of the primitive kidneys (Fig. 194, A). After 

 they have attained a considerable size, a large anastomosis is formed between 

 them ventral to the aorta and just caudal to the omphalomesenteric (superior 

 mesenteric) artery (Tig. "194, B). In the meantime, a branch of the ductus 



" C ^A^C^^cr^^ (*L fVM-A*-XX 



Int. jugular 

 (ant. cardinal; 



Innominate (right) 



Sup. vena cava**"** 



Azygos 

 (post, cardinal) 



"'Ext. jugular 

 Subclavian 



Innominate (left) 

 Coronary sinus 



Accessory 

 hemiazygos 



Hemiazygos 



FIG. 196. Diagram of final stage in the development of the superior vena cava 

 and the azygos vein. (Compare with Fig. 195.) 



venosus (see p. 229) grows caudally through the dorsal part of the liver and 

 the mesentery, and joins the right subcardinal vein a short distance cranial 

 to the above mentioned anastomosis (Fig. 194, A and B). This branch 

 forms the proximal part of the inferior vena cava. At the same time, also, 

 each subcardinal forms a direct connection with the corresponding cardinal 

 at a point opposite the first anastomosis; consequently the inferior vena 

 cava, the subcardinals and the cardinals are all in direct communication 

 (Fig. 194, B). Thus two ways are formed by which the blood may return 

 to the heart: It may return via the cardinals and ducts of Cuvier, and via 

 the inferior vena cava. (^W 



It is obvious that while these conditions exist, that is, while the mesonephros is func- 

 tional, and blood is carried to it by the cardinal veins and from it by the subcardinal veins, 

 there is a true renal portal system. The blood from the body walls and lower extremities 



