THE PHYSIOLOGY OF THE BLOOD. 
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cord. They ramify through the foetal portion of the placenta for 
its growth and support, terminating in capillary tubes, and some 
minute origins of the veins. The umbilical vein takes its origin 
from the terminating capillaries of the arteries, and the cells of 
the placenta ; ramifying through the membranes, and constantly 
uniting until they terminate in two principal veins, which 
coalesce in the cord, and form one vessel. This takes its course 
through the umbilicus, traversing the inferior part of the abdo- 
men, between the peritoneum and abdominal muscles, to the 
sternum, where it makes a curve upward and forward on the pos- 
terior surface of the diaphragm, between the layers of the broad 
ligament of the liver, until it reaches the middle lobe, by the 
side of which it passes to gain the vena portae, where, in the 
horse, it terminates. 
In the ruminantia, and those animals with a gall-bladder, when 
it reaches the liver it bifurcates ; one part goes to the posterior 
cava, and the other to the vena portae. That part of the vessel 
going to the vena cava is called the ductus venosus, the phy- 
siology of which I am unable to define. It appears evident that 
all the blood does not need purification, and consequently passes 
directly into the circulation. Having ramified through the 
liver, it is returned by the hepatic veins into the posterior cava, 
which, in the foetus, bifurcates on entering the right auricle, and 
great part of the blood passes through an opening into the left 
auricle, called the foramen ovale, which is said to be immediately 
in the septum auricularum. Anatomical investigation proves it 
to be a direct bifurcation of the cava, as it enters the auricle, at 
which opening there is a valve to direct the current of blood into 
the left auricle, and prevent its returning. 
This blood is the purest in the foetus, and mixes with that 
coming from the lungs, which is very impure. One part goes, by 
means of the anterior aorta and its branches, to supply the head 
and anterior extremities for their support and growth ; the other 
passes by the posterior aorta, and, just at its curves, mixes with 
the blood coming from the pulmonary artery through the ductus 
arteriosus, and goes to the posterior parts of the body for their 
support, a part of which is conveyed, by means of the umbilical 
arteries, for the formation and support of the foetal portion of the 
placenta. As a strong proof of this, the blood is as pure that 
passes by them as that going for the formation of some of the 
posterior parts of the foetus. The other portion of the blood 
coming from the posterior cava is received into the right auricle, 
and thence propelled into the right ventricle with that coming 
from the anterior cava arul other secretions ; all which are mix- 
ed and propelled into the pulmonary artery, where some goes 
