THORAX AND ABDOMEN OF THE HORSE 195 



blood is injected from the atrium into the left ventricle and thence into 

 the aorta. By the branches of the aorta it is distributed to all parts of 

 tiie body, except the lungs, as it is in the adult ; but from each hypo- 

 gastric artery there arises an umbilical artery that leaves the body of 

 the embryo by the umbilicus and ends in the placenta. 



It is apparent that the purest and most liigldy oxygenated blood in 

 the embryo, as v^^ell as that richest in nutritive material, is that carried 

 by the umbilical vein to the liver, where it is mixed with the portal 

 venous blood. A further mixture with impure blood occurs when that 

 of the caudal vena cava meets the pulmonary blood in the left atrium. 

 A final contamination occurs in the aorta where the ductus arteriosus 

 adds the blood that has been gathered by the cranial vena cava and 

 transmitted through the right heart into the pulmonary artery. The 

 aorta of the embryo, therefore, contains blood that is most heavily 

 loaded with waste products and poorest in oxygen and nutritive 

 material. 



The changes that take place in the circulation at birth are as 

 follows: — With the rupture of the umbilical cord blood ceases to flow 

 through the umbilical arteries and vein. The former become the 

 rounded cords (round ligaments) found in the free margin of the lateral 

 umbilical folds of the urinary bladder of the adult, while the remains of 

 the umbilical vein constitutes the round ligament of the liver. With 

 the establishment of respiration the lungs are distended, and the 

 branches of the pulmonary artery become sufficiently large to permit the 

 passage through the lungs of all the blood ejected from the right 

 ventricle. Blood ceases to flow through the ductus arteriosus, which 

 loses its lumen and becomes the ligamentum arteriosum. At birth, 

 also, the foramen ovale closes, and the blood carried to the heart by the 

 caudal vena cava is then compelled to enter the right atrium. In those 

 animals in which there is a ductus venosus in the embryo, its lumen is 

 occluded at birth, and a ligamentum venosum is produced. 



13* 



