THE COUKSE OF THE CIRCULATION. 585 



which returns blood from the intestine, and is formed in part 

 from the vitelline veins of the earlier stages. 



On reaching the liver, the blood has two alternative routes 

 open to it, by either of which it can reach the posterior vena 

 cava. Part of the blood is conveyed by the afferent hepatic 

 vessels into the substance of the liver, from which it is returned 

 by the efferent hepatic vessels, or hepatic veins, to the posterior 

 vena cava ; the greater part, however, continues straight on- 

 wards through the wide ductus venosus, and so reaches the 

 posterior vena cava without having traversed the liver. 



The blood brought back to the heart by the posterior vena 

 cava is thus derived very largely from the allantoic vein, and 

 in part from the renal veins ; it is therefore purer as regards 

 gaseous constituents, and freedom from nitrogenous excretory 

 matters, and is richer in nutrient matters, than the blood 

 returned by the anterior vena cava ; and the blood in the 

 .anterior and in the posterior vena? cavae may consequently be 

 contrasted as venous and arterial respectively. 



The venous blood brought to the right auricle by the ante- 

 rior vena cava passes, on the auricular contraction, into the 

 right ventricle. From the ventricle it is driven along the pul- 

 monary trunk (Fig. 246, RW) ; a small portion passes along the 

 pulmonary arteries, AP, to the lungs, but as the lungs are in an 

 unexpanded condition there is considerable resistance to the 

 entrance of blood into the pulmonary vessels, and only an 

 insignificant portion of the stream takes this path. Nearly the 

 whole of the venous blood in the pulmonary trunk passes along 

 the ductus arteriosus (Fig. 246, A.o) to the dorsal aorta, down 

 which it courses to the bifurcation of the aorta into the two 

 common iliac arteries ; then down these latter, and partly along 

 the external iliac arteries to the hind limbs, but mainly along 

 the allantoic arteries to the placenta, where it gains nutrient 

 matter and oxygen, and from which it is returned to the foetus 

 by the allantoic vein. 



The arterial blood brought to the right auricle by the 

 posterior vena cava does not really enter the cavity of the right 

 .auricle, but is directed at once, by the Eustachian valve, through 

 the foramen ovale into the left auricle, which also receives the 

 very small quantity of blood returned from the lungs by the 

 pulmonary veins. From the left auricle the blood passes into 



