THE rCETAL CIRCULATION 607 



internal iliac arteries and pass downwanl and forward in the umbilical folds of peri- 

 toneum on either side of the l)ladder 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 foetal 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 termetl 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 antl passes for- 

 ward along the alxlominal 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. 



In the ox and dog some of the blood in the umbilical vein is conveyed directly 

 to the vena cava by the ductus venosus (Arantii). This vessel is given off within 

 the liver from a venous sinus formed by the confluence of the portal and unil)ili('al 

 veins and passes directly to the posterior vena cava. 



The foramen ovale is an opening in the septum between the atria, by which the 

 latter communicate with each other. It is guarded by a valve (\'alvula foraminis 

 ovalis) which prevents the l)lood from passing from the left atrium to the right. 

 After birth the foramen soon closes, but this part of the septum remains membran- 

 ous, and there is a deep fossa ovalis in the right atrium which indicates the posi- 

 tion of the former opening. In some cases the foramen persists to a variable 

 extent in the adult without a|)parent disturbance of the circulation. 



The pulmonary circulation is very 1 mited 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 which go to the 

 lungs antl joins the left side of the aortic arch. After birth the pulmonary circula- 

 tion undergoes promptly an enormous increase and the ductus is rapidly trans- 

 formed into a fibrous cord — the ligamentum arteriosum. - 



The only arterial blood in the f(ctus 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 carrietl 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- 

 ried into the right atrium by the posterior vena cava passes largely, if not entirely, 

 through the foramen ovale into the left atrium, while the blood flowing into it 

 through the anterior vena cava passes into the right ventricle. On this basis the 

 blood received by the left atrium consists chiefly of mixed blood from the posterior 

 vena cava, since the small amount of blood conveyed by the pulmonary veins is 

 venous. This mixeil blood passes into the left ventricle and is forced into the 

 systemic arteries. The venous blood from the anterior part of the body and the 

 thoracic limbs is conveyed by the anterior vena cava to the right atrium, passes 

 into the right ventricle, and is forced into the pulmonary artery. A small amount 

 is carried to the lungs, but the bulk of it passes by the ductus arteriosus into the 

 aorta behind the point of origin of the brachiocephalic trunk (anterior aorta), 

 and is carried to the posterior part of the body, a large part passing by the umbilical 

 arteries to the placenta. 



