704 BLOOD-VASCULAR SYSTEM OF THE HORSE 
lie behind the origin of the gastrocnemius and between the biceps femoris and semi- 
tendinosus at the division of the posterior femoral artery into its primary branches 
(Fig. 580). They receive the deep lymph vessels of the distal part of the limb. 
Their efferent vessels chiefly follow the course of the femoral vessels to the deep 
inguinal glands, but one or two ascend in company with a vein along the great 
sciatic nerve and may enter an ischiatic gland or accompany the internal pudic 
vein and join the internal iliac glands. 
_ Several superficial lymph vessels ascend with or near the medial metatarsal and saphenous 
veins, enter the femoral canal, and end in the deep inguinal glands. 
THE FETAL CIRCULATION 
The blood of the foetus is oxygenated, receives nutrient matter, and gives off 
waste matter by close contiguity with the maternal blood in the placenta. The 
chief differences in the blood-vascular system as compared with that which obtains 
after birth are correlated with this interchange. 
The umbilical arteries, right and left, are large vessels which arise from the 
internal iliac arteries and pass downward and forward in the umbilical folds of peri- 
toneum on either side of the bladder to the umbilicus. Here they are incorporated 
with the umbilical vein and the urachus in the umbilical cord, ramify in the allan- 
tois, and end as the capillaries of the fatal placenta. They conduct the impure 
blood to the placenta. After birth these vessels retract with the bladder to the 
pelvic cavity; their lumen becomes greatly reduced and the wall thickened so 
that they are cord-like and are usually termed the round ligaments of the bladder. 
The umbilical vein receives the oxygenated blood from the placenta. Its 
radicles converge to form in the horse a single large trunk which separates from the 
other constituents of the umbilical cord on entering the abdomen and passes for- 
ward along the abdominal floor in the free border of the falciform ligament of the 
liver. It enters the latter at the umbilical fissure and joins the portal vein, so 
that the blood conveyed by it passes through the capillaries of the liver before 
entering the posterior vena cava. The vein contains no valves. 
In the ox and dog some of the blood in the umbilical vein is conveyed directly 
to the vena cava by the ductus venosus. This vessel is given off within the liver 
from a venous sinus formed by the confluence of the portal and umbilical veins 
and passes directly to the posterior vena cava. 
The foramen ovale is an opening in the septum between the atria of the heart, 
by which the latter communicate with each other. It is guarded by a valve (Val- 
vula foraminis ovalis) which prevents the blood from passing from the left atrium 
to the right. After birth the foramen soon closes, but this part of the septum 
remains membranous, and there is a deep fossa ovalis in the right atrium which 
indicates the position of the former opening. In some cases the foramen persists 
to a variable extent in the adult without apparent disturbance of the circulation. 
The pulmonary circulation is very limited in the foetus, and most of the blood 
which enters the pulmonary artery passes through the ductus arteriosus to the 
aorta. This vessel is larger than the divisions of the pulmonary artery which go to 
the lungs, and joins the left side of the aortic arch. After birth the pulmonary 
circulation undergoes promptly an enormous increase and the ductus is rapidly 
transformed into a fibrous cord—the ligamentum arteriosum. 
The only arterial blood in the feetus is that carried by the umbilical vein. 
This blood is mixed in the liver with the venous blood of the portal vein, and after 
passing through the capillaries of the liver is carried by the hepatic veins to the 
posterior vena cava. The latter receives also the venous blood from the posterior 
part of the trunk and the pelvic limbs. It is generally believed that the blood car- 
