THE F(ETAL CIRCULATION. 767 



proper aorta and the pulmonary artery. The septum of 

 the auricles is developed from a semilunar fold, which 

 extends from above downwards. In man, the septum 

 between the ventricles, according to Meckel, begins to be 

 formed about the fourth week, and at the end of eight 

 weeks is complete. The septum of the auricles, in man 

 and all animals which possess it, remains imperfect 

 throughout foetal life. When the partition of the auricles 

 is first commencing, the two venae cavse have different re- 

 lations to the two cavities. The superior cava enters, as 

 in the adult, into the right auricle ; but the inferior cava 

 is so placed that it appears to enter the left auricle, and 

 the posterior part of the septum of the auricles is formed 

 by the Eustachian valve, which extends from the point of 

 entrance of the inferior cava. Subsequently, however, the 

 septum, growing from above downwards, becomes directed 

 more and more to the left of the vena cava inferior. 

 During the entire period of fcetal life, there remains an 

 opening in the septum, which the valve of the foramen 

 ovale, developed in the third month, imperfectly closes. 



Circulation of Blood in the Fcetus. 



The circulation of blood in the foetus is peculiar, and 

 differs considerably from that of the adult. It will be 

 well, perhaps, to begin its description by tracing the 

 course of the blood, which, after being carried out to 

 the placenta by the two umbilical arteries, has returned, 

 cleansed and replenished, to the foetus by the umbilical 

 vein. 



It is at first conveyed to the under surface of the liver, 

 and there the stream is divided, a part of the blood 

 passing straight on to the inferior vena cava, through a 

 venous canal called the ductus vew sus, while the remainder 

 passes into the portal vein, and reaches the inferior vena 

 cava only after circulating through the liver. Whether, 

 however, by the direct route through the ductus venosus 



