328 



THE HEAET. 





and the ventricular portion becomes the larger part of the heart. As to the ventricles 

 themselves, the right is at first the smaller ; afterwards it becomes the larger of the 

 two, and at birth their size is about equal. In the right ventricle the infundibulum 

 is at first less marked than afterwards. 



Structure. For a time the walls of the ventricles are, comparatively speaking, 

 very thick, and the thickness of both is nearly the same. In approaching the full 

 period, however, the left begins to be the thicker of the two. But the two chief 

 differences in the internal structure of the foetal heart from that of the adult are the 

 communication which exists between the two auricles by the foramen ovale, and the 

 large size of the Eustachian valve. 



The large oval orifice named the foramen ovale is placed at the lower and back 

 part of the auricular septum, and is said to attain its greatest size at the sixth month. 

 It becomes gradually occluded by a valvular fold already alluded to, which ascends 

 from below and behind, and rises up on the left side of the rim of the foramen ovale. 

 This rim becomes continuous at the sides with the valve, but above its free margin, 

 which is concave and turned upwards, the foramen is left open. At length the valve 

 passes for some distance beyond the upper part of the foramen; and still, owing to its 

 position on the left side of the opening, it permits the passage of blood from the right 

 to the left auricle. In the reverse direction, however, it closes the opening, and no 

 blood can pass. 



Fig. 247. 



Fig. 247. VIEW OF THE POSTERIOR AND LEFT 



SURFACE OF THE HEART OF A FffiTUS OF FOUR 



MONTHS, THE LEFT AURICLE BEING OPENED 

 (from Kilian). 



a, left auriculo-ventricular orifice ; c, inferior 

 vena cava, through which a probe b, is passed 

 from below, and thence by the foramen ovale 

 into the left auricle ; e, left auricular appendage 

 laid open ; o, valve of the foramen ovale seen 

 to be attached to the left side of the annulus 

 ovalis of the septum. 



The pulmonary artery of the foetus, in 

 leaving the right ventricle, first gives off 

 the branch to the right lung, and then 

 appears to divide into its left branch and 

 the short but wide tube named ductus 

 arteriosus. This vessel, which is nearly as 

 wide as the pulmonary artery itself, is of the 

 thickness of a goose-quill at the time of birth, 

 and about half an inch long. It conducts the 

 chief part of the blood of the right ventricle 

 into the aorta, which it joins obliquely within 

 the termination of the arch, a little beyond 

 the origin of the left subclavian artery. 



Besides the usual branches of the descending aorta intended to supply the abdominal 

 viscera and the lower limbs, two large vessels, named hypogastric or umbilical arteries, 

 are prolonged from the common iliacs, and, passing out of the abdomen, proceed along 

 the umbilical cord, coiling round the umbilical vein, to reach the placenta. The com- 

 mencement of each of these vessels afterwards forms the trunk of the corresponding 

 internal iliac artery, and, from their size, they might be regarded in the foetus as the 

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

 placenta the blood is returned by the umbilical vein, which, after entering the 

 abdomen, communicates by one branch with the portal vein of the liver, and sends 

 another, named ductus venosus, to join the vena cava inferior, as will be more fully 

 described in the account of the vessels of the liver. 



Course of the blood in the foetus. The right auricle of the foetal heart receives its 

 blood from the two vense cavee and the coronary vein. The blood brought by the 

 superior cava is simply the venous blood returned from the head and upper half of 



