544 



VEINS OF THE CHICK. 



and the yolk-sack dwindles, this state of things is reversed, and the less 

 conspicnovis vitelline appears as a branch of the larger allantoic vein. 



On the third day the blood returning from the walls of the intestine 

 is insignificant in amount. As however the intestine becomes more and 

 more developed, it acquires a distinct venous system, and its blood is 

 returned by veins which form a trunk, the mesenteric vein (fig. 372, M), 

 falling into the vitelline vein at its junction with the allantoic vein. 



These three great veins, in fact, form a lai'ge common trunk, which 

 enters at once into the liver, and which we may now call the portal vein 

 (fig. 372, FV). Tliis, at its entrance into the liver, partly breaks up into 

 the vence adveJtentes, and partly continues as the ductus venosus {DV) 

 straight through the liver, emerging from which it joins the vena cava 

 inferior. Before the establishment of the vena cava inferior, the vena' 

 revehentes, cari-ying back the blood which circulates through the hepatic 

 capillaries, join the ductus venosus close to its exit from the liver. By 

 the time however that the vena cava has become a large and important 

 vessel it is found that the venai revehentes, or as we may now call them 

 the hepatic veins, have shifted their embouchment, and now fall directly 

 into that vein, the ductus venosus making a separate junction rather higher 

 up (fig. 372, IIP). 



This state of things continues with but slight changes till near the end 

 of incubation, when the chick begins to breathe the air iu the air-chamber 

 of the shell, and respiration is no longer carried on by the allantois. Blood 

 then ceases to flow along the allantoic vessels ; they become obliterated. 

 The vitelline vein, which as tlie yolk becomes gradually absorbed propor- 

 tionately diminishes in size and importance, comes to appear as a mere 

 branch of the portal vein. The ductus venosus becomes obliterated ; and 



Fig. 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 blooi 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 remain- 

 ing as the azygos vein. The coronary vein remains as the last remnant of the left 

 superior vena cava. 



