718 A MANUAL OF VETERINARY PHYSIOLOGY 



early embryonic life. The placenta acts as the foetal respiratory 

 and food-absorbing organ. Impure blood that has circulated 

 through the tissues of the developing young is brought to the 

 placenta by the umbilical arteries, these acting to the foetus 

 as the pulmonary arteries to the adult. After an interchange 

 of gases and a renewal of food supply, the blood is carried away 

 to the foetus by means of the umbilical vein, or veins, found in 

 the cord. The vein enters the body at the navel or umbilicus, 

 passes forward along the floor of the abdomen, reaches the 

 falciform ligament of the liver, travels along the free edge of 

 that structure, and empties itself into the portal vein. After 

 birth the remains of the umbilical vein are found as a thickening 

 at the free edge of the falciform ligament, and form the round 

 ligament of the liver. In ruminants the umbilical veins are two 

 in number, but they unite to form a single vessel on entering the 

 body. The vessel thus formed passes along the abdominal floor 

 towards the falciform ligament to occupy the same position as 

 in other animals, but before reaching it, it detaches a large 

 branch, the ductus venosus (Fig. 253, d.v.), which passes upwards 

 to join the posterior vena cava. After the blood has circulated in 

 the liver it leaves by the hepatic trunks, and is poured into the 

 posterior vena cava, where it meets with the blood in that 

 vessel, and is thus conducted to the heart. In the horse the 

 whole of the foetal blood passes via the portal vein through 

 the liver before reaching the heart. In ruminants, part of the 

 blood passes through the liver, and part goes direct to the 

 systemic circulation of the foetus through the ductus venosus. 



In the foetal heart the cavities of the right and left auricles 

 are in communication by means of a foramen, the foramen 

 ovale. This opening in many animals is provided with a valve, 

 the Eustachian, that stretches from the mouth of the posterior 

 vena cava to the annulus or thickened border of the foramen 

 ovale ; it is absent from the heart of the foetal horse and pig. 

 The function of this valve is to direct the blood-stream into the 

 left auricle ; the blood in this way gets into the left auricle, 

 passes into the left ventricle, and thence into the aorta. The 

 greater portion is driven into the vessels that supply the head, 

 neck, and fore-limbs (anterior aorta and branches), and is 

 conveyed to the head and anterior portion of the body ; the 

 remainder passes backwards in the posterior aorta. The head, 

 it will be noticed, receives almost pure blood. After the fluid 

 has circulated in this part of the body, it is returned to the right 

 auricle of the heart by the anterior vena cava. From the right 

 auricle it passes to the right ventricle, and from this cavity it 

 is pumped into the pulmonary artery. The lungs, however, are 

 not functional, and are more or less solid organs ; consequently 



