802 TIIE FCET-iL CLRCULATIOX. 



the umbilical vein to reach the placenta. The commencement of each of these 

 vessels afterwards forms the tmnk of the corresponding- internal iliac artery, 

 but, from their size, they might be regarded in the fojtus as the continuations 

 of the common iliac arteries into which the aorta divides. From the placenta 

 the blood is returned by the umbilical vem. which, after entering the abdomen, 

 communicates by one branch with the portal vein of the liver, and is continued by 

 another, named ductus rcnosus, into one of the hepatic veins, through which it 

 joins the main stem of the vena cava inferior. 



Course of the Blood in the Poetus. — The right auricle of the foetal heart 

 receives its blood from the two venre cavfe and the coronary vein. The blood 

 brought by the superior cava is simply the venous blood returned from the head 

 and upper half of the body ; whilst the mferior cava, which is considerably 

 larger than the superior, conveys not only the blood from the lower half of the 

 body, but also that which is returned from the placenta through the umbilical 

 vein. This latter stream of blood reaches the vena cava inferior, partly by a 

 direct passage — the ductus venosus, and partly by the hepatic veins, which bring 

 to the vena cava inferior all the blood circulating through the liver, whether 

 derived from the supply of placental blood entering by the umbilical vein, or 

 proceeding from the vena portfe or hepatic artery. 



The blood of the superior vena cava, descending in front and to the right 

 of the Eustachian valve, and mixed with a small portion of that from the 

 inferior cava, passes on into the right ventricle, and is thence propelled into the 

 trunk of the pulmonary artery. A small part of it is then distributed through 

 the branches of that vessel to the lungs, and retuins by the pulmonary veins to 

 the left auricle ; but, as these vessels remain comparatively undilated up to the 

 time of birth, by far the larger part passes through the ductus arteriosus into the 

 dorsal aorta, entering that vessel beyond the place of origin of the arteries of the 

 head and upper limbs, and, mixed perhaps with a small quantity of the blood 

 flowing into the aorta from the left ventricle, is distributed in part to the lower 

 half of the body and the viscera, and in part is conveyed along the umbilical 

 arteries to the placenta. From these several organs it is returned by the vena 

 cava inferior, the venaj portsB, and the umbilical vein : and, as already noticed, 

 reaches the right auricle through the trank of the inferior cava. 



Of the blood entering the heart by the inferior vena cava, only a small part is 

 mingled with that of the superior cava, so as to pass into the right ventricle ; 

 by far the larger portion, directed by the Eustachian valve through the foramen 

 ovale, flows from the right into the left auricle, and thence, together with the 

 small quantity of blood returned from the lungs by the pulmonary veins, passes 

 into the left ventricle, from whence it is sent into the arch of the aorta, to be 

 distributed almost entu-ely to the head and upper limbs. A small portion of it, 

 may, however, flow on into the descending aorta, and join the fuller stream of 

 blood from the ductus arteriosus. From the upper half of the body the blood is 

 returned by the branches of the superior cava to the right auricle, from which 

 its course into the right ventricle and pulmonary trunk has been already traced. 



There is probably a considerable difference in the early and more advanced 

 stages of foetal life, m the distribution of the stream of blood entering the heart 

 by the vena cava inferior. In the early stages, a lai-ge part of the cuiTcnt being 

 directed into the left, but in the three last months, and as the foetus approaches 

 maturity, more and more of the blood of the inferior cava joins the stream from 

 the superior cava ; and, indeed, the coui-se of the blood, and the relative position 

 of the veins, as well as other original peculiarities of the foetal heart, become 

 gradually altered, in preparation, as it were, for the important changes which 

 take place at birth. It seems also probable that very little of the blood pro- 

 pelled from the left ventricle passes into the descending aoi-ta beyond the 

 ductus arteriosus diu-ing those months of foetal life in which the peculiarities of 

 the circulation are most complete. 



From the preceding account of the course of the blood in the foetus, it will be 

 seen that, whilst the modified blood from the placenta is principally conveyed to 

 the upper or cephalic half of the foetus, the lower half of the body is chiefly 

 supplied with the blood which has already circulated through the head and upper 



