574 THE VASCULAR SYSTEMS 



arteriosus into the commencement of the descending aorta, where it becomes 

 mixed with the blood transmitted by the left ventricle into the aorta. Through 

 this vessel it descends to supply the lower extremities and viscera of the abdomen 

 and pelvis, the chief portion being, however, conveyed by the umbilical arteries 

 to the placenta. 



From the preceding account of the circulation of the blood in the fetus it will 

 be seen 



1. That the placenta serves the purposes of nutrition, respiration, and excretion, 

 receiving the impure blood from the fetus, and returning it charged with addi- 

 tional nutritive material. 



2. That nearly the whole of the blood of the umbilical vein traverses the liver 

 before entering the inferior vena cava; hence the large size of this organ, especially 

 at an early period of fetal life. 



3. That the right auricle is the point of meeting of a double current, the blood 

 in the inferior vena cava being guided by the Eustachian valve into the left auricle, 

 while that in the superior vena cava descends into the right ventricle. At an early 

 period of the fetal life it is somewhat probable that the two streams are distinct, 

 for the inferior vena cava opens almost directly into the left auricle, and the Eusta- 

 chian valve would tend to exclude the current along the vein from entering the 

 right ventricle. At a later period, as the separation between the two auricles 

 becomes more distinct, it seems more probable that mixture of the two streams 

 must take place. 



4. The pure blood carried from the placenta to the fetus by the umbilical vein, 

 mixed with the blood from the portal vein and the inferior vena cava, passes almost 

 directly to the arch of the a,orta, and is distributed by the branches of that vessel 

 to the head and upper extremities; hence the large size and advanced development 

 of those parts at birth. 



5. The blood contained in the descending aorta, largely derived from that 

 which has already circulated through the head and upper limbs, .together with a 

 small quantity from the left ventricle, is distributed to the lower extremities; 

 hence the small size and less advanced development of these parts at birth. 



Changes in the Vascular System at Birth. At birth, when respiration is 

 established, an increased amount of blood from the pulmonary artery passes 

 through the lungs, which now perform their office as respiratory organs, and at 

 the same time the placental circulation is cut off. Soon after birth the foramen 

 ovale is closed by the valvular edge being pressed against the annulus ovalis, the 

 pressure being due to respiration, which increases the pressure in the left auricle. 

 The structures fuse, and closure is usually complete by about the tenth day after 

 birth. The valvular fold above mentioned becomes adherent to the margins of 

 the foramen for the greater part of its circumference, but above a slit-like opening 

 is left between the two auricles which sometimes remains persistent. 



The ductus arteriosus begins to contract immediately after respiration is estab- 

 lished, usually becomes completely closed from the fourth to the tenth day, and 

 ultimately degenerates into an impervious cord which serves to connect the left 

 pulmonary artery to the arch of the aorta. 



Of the umbilical or hypogastric arteries, the portion continued on to the bladder 

 from the trunk of the corresponding internal iliac remains pervious as the superior 

 vesical artery, and the part extending from the side of the bladder to the umbilicus 

 becomes impervious between the second and fifth days after birth, and projects 

 as a fibrous cord toward the abdominal cavity, carrying-on it a fold of peritoneum. 



The umbilical vein and the ductus venosus become impervious between the second 

 and fifth days after birth, and ultimately dwindle to fibrous cords, the former 

 becoming the round ligament of the liver, the latter the ligamentum venosum of 

 the liver. 



