VIVIPAROUS ANIMALS. 405 
ed directly to the liver, the other portion goes to the pul- 
monic auricle by the inferior vena cava. The septum be- 
tween the pulmonic and systemic auricle is at this time in- 
complete, there being a valvular aperture termed foramen 
ovale. Through this opening part of the blood in the pul- 
monic auricle escapes into the systemic auricle, and the re- 
maining portion, passing into the pulmonary artery, instead 
of going directly to the lungs, is conveyed to the aorta, by 
a passage termed ductus arteriosus. The blood thus expelled 
by the systemic ventricle into the aorta, is conveyed te the 
different parts of the body, but .a great part is conveyed 
back to the placenta by the umbilical arteries, which take 
their rise from the iliacs. 
Although the foetus is thus nourished by blood derived 
from the mother, the communication is so indirect, that no 
injection into the uterine vessels passes into the foetus, and 
no injection of the foetus reaches the vessels of the mother. 
The two sets of vessels, therefore, do not anastomose. The 
imperfect connection of the vessels is still farther demon- 
strated by the superior rapidity of circulation in the foetus, 
when compared with the mother, the heart of the former, in 
the human subject, beating about one hundred and twenty 
times ina minute. The circulation of the foetus even con- 
tinues for some time after the death cf the mother. But 
however indirect the communication between the two circu- 
lating systems, the condition of the mother still exercises a 
considerable influence over the growth of the foetus. A 
derangement in the flow of her blocd, by dimmishing the 
quantity sent to the placenta, must consequently reduce the 
supply to the umbilical veins *. A diseased state of the 
mother is likewise frequently communicated to the foetus. 
* Where more foetuses than usual are generated in the uterus, the supply 
of nourishment being thus divided, they seldom reach the ordinary size, and 
