CH. LVlll.] THE F(ETAL CIRCULATION. 839 



The upper portions of the left posterior cardinal vein remain as 

 the left superior intercostal and vena azygos minor. 



The azygos veins receive the intercostal veins as shown in the 

 diagrams. 



These views of the origins of the veins are chiefly derived from the 

 observations of Rathke and Gegenbaur. They have been generally accepted 

 by embryologists. Hochstetter, however, has more recently stated that some 

 modification of these views is necessary. According to him, the right 

 common iliac vein and the portion of the inferior vena cava below the renal 

 vein are parts of the right cardinal vein, and the greater part of the left 

 common iliac is the transverse iliac vein, a vein which grows across at a 

 lower level than the transverse azygos. According to these views, the low er 

 part of diagram 6570 would have to be altered as in fig. 658. 



CIRCULATION OF BLOOD IN THB FOSTUS. 



The circulation of blood in the foetus differs considerably from 

 that of the adult. It will be well, perhaps, to begin its descrip- 

 tion 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 venosus, 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 or by the roundabout way through the liver, all 

 the blood which is returned from the placenta by the umbilical 

 vein reaches the inferior vena cava at last, and is carried by it 

 to the right auricle of the heart, into which cavity is also pouring 

 the blood that has circulated in the head and neck and arms, and 

 has been brought to the auricle by the superior vena cava. It 

 might be naturally expected that the two streams of blood would 

 be mingled in the right auricle, but such is not the case, or only 

 to a slight extent. The blood from the superior vena cava, the 

 less pure fluid of the two passes almost exclusively into the right 

 ventricle, through the auriculo-ventricular opening, just as it does 

 in the adult ; while the blood of the inferior vena cava is directed 

 by the fold of the lining membrane of the heart, called the Eutta- 

 cMan valve, through the foramen ovale into the left auricle, whence 

 it passes into the left ventricle, and out of this into the aorta, and 

 thence to all the body, but chiefly to the head and neck. The 

 blood of the superior vena cava, which, as before said, passes into 



