CIRCULATION IN FOETUS 233 



and over the dorsal aspect of the future hand or foot to the pre-axial border, there 

 to become continuous with a smaller and temporary channel, the radial or tibial 

 vein respectively. These latter veins are replaced by new (secondary) veins which 

 become the radial and cephalic in the upper arm (opening at first into the external 

 jugular), and the long saphenous in the lower limb. The primary ulnar vein persists 

 in the upper arm as the basilic, axillary, and subclavian veins ; the primary 

 fibular, on the other hand, persists in the leg only, as the short saphenous. The 

 deep veins which accompany the arteries are later formations. 1 



PECULIAKITIES OF THE FCETAL ORGANS OF CIRCULATION. 



It may be useful here to recapitulate shortly the peculiarities of structure 

 existing in the advanced stage of the formation of the foetal organs of circulation, 

 with reference to their influence in determining the course of the blood during 

 intra-uterine life, and the changes which occur in them upon the establishment 

 of pulmonary respiration at birth. 



The foramen ovale has the form of a free^ oval opening bounded by the 

 septum secundum, and guarded on the side of the left auricle by a valvular plate 

 derived from the septum primum, so that the blood can only pass from the right 

 into the left auricle, not in a contrary direstion. 



The Eustachian valve constitutes a crescentic fold of the lining structure of 

 the heart, which is so situated as to direct the blood entering the auricle by the 

 inferior cava towards the opening of the foramen ovale. 



The ductus arteriosus establishes a communication between the main 

 pulmonary artery and the aorta, by which the blood from the right ventricle is 

 carried mainly into the dorsal aorta. 



The two large hypogastric or umbilical arteries, prolonged from the iliac 

 arteries, passing out of the body of the foetus, proceed along the umbilical cord, 

 to be distributed in the foetal portion of the plicenta. From the placenta the 

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

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

 named ductus venosus, into one of the hepatic veins, through which it joins the 

 main stem of the vena cava inferior. 



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

 receives blood from the two venae cavse and the coronary sinus. The blood brought 

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

 half of 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 returned from the placenta and the liver. 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 that organ by the umbilical vein, or proceeding from 

 the vena portae or hepatic artery. 



The blood of the superior vena cava is believed to pass through the right 

 auricle into the right ventricle, whence it is propelled into the trunk of the 

 pulmonary artery. A small part is distributed through the branches of that vessel 

 to the lungs, and returns by the pulmonary veins to the left auricle ; but, as these 

 vessels remain small up to the time of birth, by far the larger part passes through 

 the ductus arteriosus into the descending aorta, and is thence distributed in part 

 to the lower half of the body and the viscera, and in part along the umbilical arteries 

 to rhe placenta. From these several organs it is returned by the vena cava inferior, 



1 See paper by Lewis, Amer. Jour, of Anat. v. 1905. 



