CIRCULATION OF BLOOD IN THE FETUS 



695 



or crushing through of that part at which its cohesion is least strong, namely, 

 where it is most burrowed and undermined by the cavernous spaces before 

 referred to. In this way it is cast off with the fetal membrane. The remain- 

 ing portion is either gradually absorbed, or thrown off in the uterine dis- 

 charges which occur at this period. A new mucous membrane is of course 

 gradually developed. 



Circulation of Blood in the Fetus. The circulation of blood in 

 the fetus differs considerably from that of the adult. 



Returning from the placenta by the umbilical vein the blood is first con- 

 veyed to the under surface of the liver, where the stream is divided a part of 

 the blood passing straight on to the inferior vena cava through a venous canal 



FIG. 503. Diagrammatic View of a Vertical Transverse Section of the Uterus at the Seventh 

 or Eighth Week of Pregnancy, c, c, c', Cavity of uterus, which becomes the cavity of the decidua, 

 opening at c, c, the cornua, into the Fallopian tubes, and at cf into the cavity of the cervix, which 

 is closed by a plug of mucus; dv, decidua vera; dr, decidua reflexa, with the sparser villi embedded 

 in its substance; ds, decidua serotina, involving the more developed chorionic villi of the commenc- 

 ing placenta. The fetus is seen lying in the amniotic sac. The umbilical cord and its vessels 

 pass up from the umbilicus to the distribution of the blood-vessels in the villi of th chorion- 

 and the pedicle of the yolk-sac the cavity between the amnion and chorion. (Allen Thomson.) 



called the ductus venosus, while the remainder passes into the portal vein and 

 reaches the inferior vena cava only after circulating through the liver. It is 

 carried by the vena cava to the right auricle of the heart, into which cavity the 

 blood is also pouring 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 



