438 GENERATION. 



greater part of the blood returned to the foetus from the pla- 

 centa passes through the liver, a relatively small quantity 

 being emptied into the vena cava by the ductus venosus. 



The vena cava ascendens, containing the placental blood 

 which has passed through the liver, the blood conveyed di- 

 rectly from the umbilical vein by the ductus venosus, and 

 the blood from the lower extremities, passes to the right au- 

 ricle. As the blood enters the right auricle, it is directed by 

 the Eustachian valve, passing behind the valve, through the 

 foramen ovale, into the left auricle. At the same time, the 

 blood from the head and the superior extremities passes down, 

 by the vena cava descendens, in front of the Eustachian valve, 

 through the right auricle, into the right ventricle. The ar- 

 rangement of the Eustachian valve is such, that the right au- 

 ricle simply affords a passage for the two currents of blood ; 

 the one, from the vena cava ascendens, through the foramen 

 ovale, passes into the left auricle and the left ventricle ; and 

 the other, from the vena cava descendens, passes through the 

 right auriculo-ventricular opening, into the right ventricle. It 

 is probable, indeed, that there is very little admixture of these 

 two currents of blood in the natural course of the foetal cir- 

 culation. Reid injected the vena cava ascendens with red, 

 and the vena cava descendens with yellow, in a foetus of 

 seven months, and found very little mixture of the two col- 

 ors in the passage of the injected material through the right 

 auricle. 1 



The blood poured into the left auricle from the vena cava 

 ascendens through the foramen ovale passes from the left 

 auricle into the left ventricle. The left auricle and the left 

 ventricle also receive a small quantity of blood from the 

 lungs, by the pulmonary veins. Thus the left ventricle is 

 filled. At the same time, the right ventricle is filled with 

 blood which has passed through the right auricle, in front of 



1 REID, Injections of the Vessels of the Fcetus, to show some of the Peculiari- 

 ties of its Circulation. Physiological Anatomical and Pathological Researches, 

 Edinburgh, 1848, p. 339. 



