IX.] THE VENOUS SYSTEM. 289 



ductus venosus making a separate junction rather higher 

 up (Fig. 90). 



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 proportionately 

 diminishes in size and importance, comes to appear as 

 a mere branch of the portal vein. The ductus venosus 

 becomes closed, remaining often as a mere ligament; 

 and hence the whole of the blood coming through the 

 portal vein flows into the substance of the liver, and 

 so by the two hepatic veins into the vena cava (Fig. 

 91, HP}. 



Previous to these changes one of the veins passing 

 from the rectum into the vena cava has given off a 

 branch which effects a junction with one of the mesen- 

 teric veins. This now forms a somewhat conspicuous 

 connecting branch between the systems of the vena 

 cava and the portal vein (Fig. 91, Cy. M.). 



All three venae cavae now fall exclusively into the 

 right auricle, and by the closure of the foramen ovale 

 the blood flowing through them is entirely shut off from 

 the left auricle, into which passes the blood from the 

 two pulmonary veins (Fig. 91, L. V.). 



Such is the history of the veins in the chick. As 

 will be seen in the second part of this work, the course 

 of events in the mammal, though in the main similar, 

 differs in some unimportant respects. 



It remains for us to speak of the changes which 

 F. & B. 19 



