THE VENOUS SYSTEM. 



659 



V.C.L-. 



cardinal or jugular vein (fig. 37 1, /), of each side, is joined by two new 

 veins : the vertebral vein, bringing back blood from the head and neck, and 

 the subclavian vein from the wing (IV}. 



On the third day the posterior cardinal veins are the only veins which 

 return the blood from the hinder part of the body of the embryo. 



About the fourth or fifth day, however, the vena cava inferior (fig. 371, 

 r.C.I.) makes its appearance. This, starting 

 from the sinus venosus not far from the heart, 

 is on the fifth day a short trunk running back- 

 ward in the middle line below the aorta, and 

 speedily losing itself in the tissues of the 

 Wolffian bodies. When the true kidneys are 

 formed it also receives blood from them, and 

 thenceforward enlarging rapidly becomes the 

 channel by which the greater part of the blood 

 from the hinder part of the body finds its way 

 to the heart. In proportion as the vena cava 

 inferior increases in size, the posterior cardinal 

 veins diminish. 



The blood originally coming to them from 

 the posterior part of the spinal cord and trunk 

 is transported into two posterior vertebral veins, 

 similar to those in Reptilia, which are however 

 placed dorsally to the heads of the ribs, and 

 join the anterior vertebral veins. With their 

 appearance the anterior parts of the posterior 

 cardinals disappear. The blood from the hind 

 limbs becomes transported directly through the 

 kidney into the vena cava inferior, without 



371. DIAGRAM OF 

 THE VENOUS CIRCULATION 

 IX THE CHICK AT THE COM- 

 MENCEMENT OF THE FIFTH 



DAY. 



H. heart; d.c. cluctus Cu- 

 vieri. Into the cluctus Cuvieri 

 of each side fall_/. the jugular 

 vein, IV. the vein from the 

 wing, and c. the inferior car- 

 dinal vein ; 6". V. sinus venosus ; 

 Of. vitelline vein ; U. allan- 

 toic vein, which at this stage 

 gives off branches to the body- 

 walls ; V.C.I . inferior vena 



forming a renal portal system 1 . cava ' liver - 



On the third day the course of the vessels from the yolk-sack is very 

 simple. The two vitelline veins, of which the right is already the smaller, 

 form the ductus venosus, from which, as it passes through the liver on its 

 way to the heart, are given off the two sets of vencs advehentes and vence 

 revehentes (fig. 371). 



With the appearance of the allantois on the fourth day, a new feature is 

 introduced. From the ductus venosus there is given off a vein which 

 quickly divides into two branches. These, running along the ventral walls 

 of the body from which they receive some amount of blood, pass to the 

 allantois. They are the allantoic veins (fig. 371, U) homologous with the 

 anterior abdominal vein of the lower types. They unite in front to form a 

 single vein, which becomes, by reason of the rapid growth of the allantois, 

 very long. The right branch soon diminishes in size and finally disappears. 

 Meanwhile the left on reaching the allantois bifurcates ; and, its two 



1 The mode in which this is effected requires further investigation. 



422 



