THE VENOUS SYSTEM. 



66l 



inferior, in that the caudal vein and posterior pelvic veins open into a 

 vessel, known as the coccygeo-mesenteric vein, which joins the portal 

 vein ; while at the same time the posterior pelvic veins are connected with 

 the common iliac veins by a vessel which unites with them close to their 

 junction with the coccygeo-mesenteric vein. 



Mammalia. In Mammals the same venous trunks are developed in 

 the embryo as in other types (fig. 373 A). The anterior cardinals or 

 external jugulars form the primitive veins of the anterior part of the body, 

 and the internal jugulars and anterior vertebrals are subsequently formed. 

 The subclavians (fig. 373 A, j), developed on the formation of the anterior 

 limbs, also pour their blood into these primitive trunks. In the lower 

 Mammalia (Monotremata, Marsupialia, Insectivora, some Rodentia, etc., 

 the two ductus Cuvieri remain as the two superior venae cavae, but more 

 usually an anastomosis arises between the right and left innominate veins, 

 and eventually the whole of the blood of the left superior cava is carried to 

 the right side, and there is left only a single superior cava (fig. 373 B and C). 



F IG - 373- DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS SYSTEM OF 



MAMMALS (MAN). (From Gegenbaur.) 



j. jugular vein ; cs. vena cava superior; s. subclavian veins; c. posterior cardinal 

 vein ; v. vertebral vein ; az. azygos vein ; cor. coronary vein. 



A. Stage in which the cardinal veins have already disappeared. Their position 

 is indicated by dotted lines. 



B. Later stage when the blood from the left jugular vein is carried into the right 

 to form the single vena cava superior ; a remnant of the left superior cava being how- 

 ever still left. 



C. Stage after the left vertebral vein has disappeared; the right vertebral 

 remaining as the azygos vein. The coronary vein remains as the last remnant of the 

 left superior vena cava. 



A small rudiment of the left superior cava remains however as the sinus 

 coronartus and receives the coronary vein from the heart (figs. 373 C, 

 cor and 374, cs). 



The posterior cardinal veins form at first the only veins receiving the 



