MIDLAND COUNTIES VETERINARY MEDICAL ASSOCIATION. 775 
animal. Taken together these are called the placental membranes. 
During the existence of the foetus in the womb its lungs are inactive, 
the blood, as I have before said, being derived from the mother, 
receives in her lungs the oxygen from the atmosphere which is so 
essential to the preservation of life, and parting with the deleterious 
carbonic acid with which it has become charged. 
It is in the outer membrane (the chorion) that the foetal blood 
meets that of the mother; and if you do not consider me tedious 
in these preliminary observations, I think it may not be uninter¬ 
esting to notice very briefly the mode in which the vital fluid 
circulates in the foetus, which is in a very different manner to the 
circulation in the adult. From the placental membranes the blood 
is sent to the foetus by the two umbilical veins; part of it passes to 
the liver, where it mingles with that from the vena portee, and after 
ramifying through the structure and capillaries of that organ, passes 
by the hepatic veins to the vena cava posterior. The remaining 
portion of the blood from the umbilical veins does not go through 
the liver, but reaches the vena cava by means of a direct channel 
called the ductus venosus, which I may remark in passing is 
believed to have no existence in the horse, consequently the whole 
of the blood circulating in the equine foetus passes through the 
liver before it reaches the vena cava. This large vessel (the pos¬ 
terior vena cava) conveys the blood to the right auricle of the heart, 
whence the greater portion passes through the foramen ovale to 
the left auricle, thence to the left ventricle ; the other portion being 
sent from the right auricle to the right ventricle, and into the pul¬ 
monary artery, part passing through that vessel to the lungs, the 
remaining portion through a channel called the ductus arteriosus 
to the posterior aorta ; that portion going to the lungs is returned 
to the left auricle of the heart, as in the adult, by the pulmonary 
veins, and from the left ventricle passes through the general 
arterial system, the internal iliac arteries giving off the umbilical, 
which return the blood to the placental membranes. The arterial 
and venous blood of the foetus, for obvious reasons, does not pre¬ 
sent the same characteristic differences as in the adult. 
Having thus given a brief and consequently very imperfect de¬ 
scription of the manner in which the young animal is developed 
and nourished in the uterus, I come to the immediate object of my 
paper, viz. To inquire into those causes whereby the beautiful pro¬ 
cess I have been attempting to describe is interrupted. A premature 
disconnection takes place between the mother and the foetus, the 
latter loses its vitality, and is cast off as an useless burden. This is 
called abortion, which is defined as the expulsion of a foetus, which 
is either already dead or is at a too early period of foetal existence 
to live. It occasionally happens that a foetus which is expelled 
before the proper period of utero-gestation has been completed does 
live; this is not true abortion, but may be more correctly called 
premature birth. Abortion may occur at any period during preg¬ 
nancy ; the causes may be divided into those which act primarily 
upon the mother and depend upon her, the maternal and the 
