66o 



VEINS OF THE CHICK. 



branches becoming large and conspicuous, there still appear to be two 

 main allantoic veins. At its first appearance the allantoic vein seems to be 

 but a small branch of the vitelline, but as the allantois grows rapidly, 

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

 spicuous 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 deve- 

 loped, 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 

 large common trunk, which enters at once 

 into the liver, and which we may now call 

 the portal vein (fig. 372, P. V}. This, at its 

 entrance into the liver, partly breaks up 

 into the vena advehentes, and partly con- 

 tinues as the ductus venosus (D.V} 

 straight through the liver, emerging from 

 which it joins the vena cava inferior. Before 

 the establishment of the vena cava inferior, 

 the venas revehentes, carrying back the 

 blood which circulates through the hepatic 

 capillaries, join the ductus venosus close to 

 its exit from the liver. By the time how- 

 ever that the vena cava has become a large 

 and important vessel it is found that the 

 venae 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 sepa- 

 rate junction rather higher up (fig. 372). 



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

 of incubation, when the chick begins to breathe the air in 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 the 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 

 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 



FIG. 372. DIAGRAM OF THE 

 VENOUS CIRCULATION IN THE 

 CHICK DURING THE LATER DAYS 

 OF INCUBATION. 



H. heart ; V.S.R. right vena 

 cava superior; V.S.L. left vena cava 

 superior. The two venas cavrc 

 superiores are the original 'ductus 

 Cuvieri,' they open into the sinus 

 venosus. J. jugular vein; Su.V. 

 anterior vertebral vein ; In. V. in- 

 ferior vertebral vein ; W. subcla- 

 vian; V.C.I, vena cava inferior; 

 D. V. ductus venosus ; P. V. portal 

 vein ; M. mesenteric vein bringing 

 blood from the intestines into the 

 portal vein ; O.f. vitelline vein ; U. 

 allantoic vein. The three last men- 

 tioned veins unite together to form 

 the portal vein ; /. liver. 



