VENOUS SYSTEM. 



545 



hence the whole of the blood coming through the portal vein flows into the 

 substance of the liver, and so by the hepatic veins into the vena cava. 



Although the allantoic (anterior abdominal) vein is obliterated in the 

 adult, there is nevertheless established an anastomosis between the portal 

 system and the veins bringing the blood from the limbs to the vena cava 

 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 bo-ly, 

 and the internal jugulars and anterior vertebrals are subsequently formed. 

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

 limb*, 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 cava?, 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). A small rudiment of the 

 left superior cava remains however as 

 the sinus coronarius and receives 

 the coronary vein from the heart 

 (figs. 373 C, cor and 374, ). 



The posterior cardinal veins form 

 at first the only veins receiving the 

 blood from the posterior port of the 

 trunk and kidneys ; and on the deve- 

 lopment of the hind limbs receive the 

 blood from them also. 



As in the types already described 

 an unpaired vena cava inferior becomes 

 eventually developed, and gradually car- 

 ries off a larger and larger portion of 

 the blood originally returned by the 

 posterior cardinals. It unites with the 

 common stem of the allantoic and vitel- 

 line veins in front of the liver. 



At a later period a pair of trunks 

 is established bringing the blood from 

 the posterior part of the cardinal veins 

 and the crural veins directly into the 

 vena cava inferior (fig. 374, if). These 

 vessels, whose development has not been 

 adequately investigated, form the com- 

 mon iliac veins, while the posterior ends 

 of the cardinal veins which j"in them 

 become the hypogastric veins (fig. 374 

 ly}. Owing to the development of the 



I 1 .. K. II. 



FIG. -".7t. I>IAI;I;AM OF THE CHIEF 

 VENOT-S TI.TINKS OK MAN. (From Ge- 

 genbaur.) 



cs. vena cava superior; s. sub- 

 clavian vein ; .//. internal jugular ; je. 

 external jugular ; a. azygos vein ; Int. 

 hemiazygos vein ; c. dotted line shew- 

 ing previous position of cardinal veins ; 

 c). vena cava inferior ; r. renal veins ; 

 77. iliac; ////. hypogastric veins; I/. 

 hepatic veins. 



The dotted lines shew the position 

 of embryonic vessels aborted in the 

 adult. 



35 



