THE FOETAL CIRCULATORY SYSTEM 



117 



Fig. 66. Diagram of a ventral 

 view of the course of the blood 

 through the heart of a rabbit 

 shortly before birth, ao, aorta; f, 

 foramen ovale; i, opening of in- 

 ferior vena cava; 1, opening of left 

 superior vena cava ; la, left atrium ; 

 Iv. left ventricle; pa, pulmonary- 

 artery-; pv, openings of pulmonary 

 veins; r, right superior vena cava; 

 ra. right atrium ; rv, right ventricle. 



The Linoxygenated blood from the 

 rep^ions in front of the heart enters 

 the right atrium through the superior 

 caval veins and, although there is no 

 partition to separate it from the 

 stream entering by the inferior caval, 

 it is mainly directed through the 

 right atrioventricular opening to the 

 right ventricle and so into the pul- 

 monary artery. The lungs being 

 non-functional until birth, however, 

 only a part of this current is carried 

 to them, the greater portion passing 

 through the wide ductus arteriosus 

 (the retained dorsal part of the left 

 sixth aortic arch) to the aorta. These 

 vessels carrying unox^^genated blood 

 appear black in the diagram. Some 

 of the mixed blood which entered the 

 aorta from the left ventricle has been 

 distributed through the carotid arteries to the head and through 

 the subclavian arteries to the anterior limbs before this final 

 admixture of unoxygenated blood through the ductus arteriosus 

 occurs, so that these anterior parts receive blood better ox\genated 

 than that which reaches the trunk and tail. At its caudal end, the 

 aorta divides into a pair of large common iliac arteries, the greater 

 part of the blood from which enters the umbilical arteries and so is 

 returned to the placenta to have its load of oxygen renewed. The 

 external iliac artery, which continues into the hind limb, is con- 

 siderably smaller in the foetus than the umbilical, and the hypogas- 

 tric (internal iliac) is smaller still. 



Radical changes in these dispositions occur at birth. The 

 placenta is suddenly lost and the flow of blood through the umbilical 

 vein ceases, this vessel rapidly degenerating to a cord of connective 

 tissue, the remains of which appear in the adult as the round 

 ligament of the liver. The wide passage through the liver, the 

 ductus venosus, also becomes obliterated so that all blood entering 

 that organ has to flow through its sinusoids to reach the hepatic 

 veins and enter the vena cava. Since no oxygenated blood now 



