THE FCETAL CIRCULATION. 329 



the body; whilst the inferior 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 

 sent back in a purified state from the placenta through the umbilical vein. This 

 latter stream of blood reaches the vena cava inferior, partly by a direct passage the 

 duetus venosus, and partly by the hepatic veins after circulating through the liver in 

 the venae portse. 



The blood of the superior vena cava, descending in front 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 returns by the pulmonary veins to the left auricle ; but by far the larger part 

 passes through the duetus arteriosus into the aorta, entering that vessel beyond the 

 place of origin of the arteries of the head and upper limbs, and, mixed probably 

 with a small quantity of the blood flowing along the aorta from the left ventricle, 

 descends partly to supply the lower half of the body and the viscera, but principally 

 to be conveyed along the umbilical arteries to the placenta. From all these parts it 

 is returned by the vena cava inferior, the venae portae, and the umbilical vein ; and, 

 as already noticed, reaches the right auricle through the trunk of the inferior cava. 



The blood of the inferior vena cava is only in small part distributed with that of 

 the superior cava ; 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, whence it is sent into the arch of the aorta, to be distri- 

 buted almost entirely to the head and upper limbs. A small portion of it, how- 

 ever, probably flows on into the descending aorta and joins the large stream of blood 

 from the duetus 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 has 

 been already traced. 



Sabatier was of opinion that no mixture of the two streams of blood from the two 

 venae cavae took place in the right auricle, but that all the blood of the inferior cava 

 passed into the left auricle and ventricle, whilst that of the superior cava reached the 

 right ventricle. He thought, however, that the two kinds of blood were intermixed at 

 the junction of the duetus arteriosus with the aorta. The entire separation of the two 

 streams of blood of the venae cavaa, as supposed by Sabatier, is not generally admitted 

 in the mature foetus ; but there is reason to believe that it does take place in earlier 

 stages. In fact, the inferior cava, as already mentioned, at first opens into the left 

 auricle, and must therefore convey its blood immediately into that cavity. 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 course of the blood, and the relative 

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

 gradually altered, to prepare the way as it were for the more 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 aorta beyond the duetus 

 arteriosus. 



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

 that, whilst the renovated 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 limbs, exhibiting 

 in this a certain analogy with the mode of circulation in the turtle and various other 

 reptiles. The larger portion of this latter stream of blood, however, is again sent out 

 of the body to be changed in the placenta. This duty is principally performed by 

 the right ventricle, which after birth is charged with an office somewhat analogous, in 

 having to propel the blood through the lungs. But the passage of the placental blood 

 is longer than that of the pulmonary, and the right ventricle of the foetus, although 

 probably aided by the left in the placental circulation, also takes a large share in the 

 systemic circulation through the lower half of the body ; and this, perhaps, may be 

 the reason why the right differs less in thickness from the left ventricle in the foetus 

 than in the adult. 



Changes after birth. The immediate changes which take place at birth consist of 

 the sudden stoppage of the placental circulation and the simultaneous commence- 



