THE CIRCULATORY OR VASCULAR SYSTEM 141 



quantity of blood transmitted by the left ventricle into 

 the aorta. Along this vessel it descends to supply the 

 lower extremities and viscera of the abdomen and pelvis, 

 the chief portion being, however, conveyed by the um- 

 bilical arteries to the placenta. 



The placenta serves the double purpose of a respira- 

 tory and nutritive organ, receiving the venous blood from 

 the fetus, and returning it again reoxygenated and charged 

 with additional nutritive material. 



Changes at Birth. At birth, when respiration is 

 established, an increased amount of blood from the pul- 

 monary artery passes through the lungs, which now per- 

 form their office as respiratory organs, and, simultane- 

 ously, the placental circulation is cut off. The opening 

 between the two auricles (foramen ovale) now becomes 

 gradually closed in, generally about the tenth day; val- 

 vular folds now form about the valvular opening or fora- 

 men; sometimes this opening is persistent and never 

 closes. 



The other parts of the ducts that furnish the circula- 

 tion from the after-birth, through the medium of the funis, 

 or cord, become obliterated and form attachments to the 

 several organs of the internal viscera. 



The illustration (Fig. 68) will give a fair idea of the 

 course of the blood to and from the after-birth, and of 

 the location of the arterial structure. 



The ductus arteriosus forms a cord that connects the 

 left pulmonary artery to the concavity of the arch of the 

 aorta. 



The umbilical arteries form ligaments to the bladder, and 

 the umbilical veins and ductus venosus become obliterated 

 to form fibrous cords. The remains of the umbilical vein 

 forms the round ligament of the liver. 



