254 REPRODUCTION ' 



but the major part of the blood leaves the fetal body by the 

 hypogastric arteries. The hypogastric arteries are branches of 

 the internal iliacs, and course along the abdomen to leave the 

 fetal body at the umbilicus, where on emerging they change 

 their names to umbilical arteries and proceed to the placenta. 

 The liver, receiving the freshest blood (from the umbilical 

 vein), is the best nourished of all the organs of the fetus. The 

 result is that the fetal liver is vastly larger in proportion than 

 the adult liver. The branches of the aorta given off to the head 

 and upper extremities distribute blood from the inferior vena 

 cava, while the ductus arteriosus, carrying the blood from the 

 superior cava and right ventricle, enters the aorta in such a 

 way that most of its blood is sent to the lower extremities, 

 abdominal organs, and umbilical arteries. In this way the 

 deoxidized blood is sent back to the placenta for the renewal of 

 its oxygen. The lower extremities are less developed than the 

 upper. There are two reasons for this : 



1. The blood contains less oxygen and nourishment. 



2. The internal iliac arteries, giving off the umbilical arteries, 

 divert a considerable portion of the blood supply from the 

 external iliacs which go to the lower extremities. 



Owing to the ductus arteriosus, but little blood goes to the 

 lungs. The amount is sufficient, however, to keep up the nutri- 

 tion of the lungs, and they have no function before birth. 



The respiratory centre in the medulla, which has been quiescent 

 because it has been well supplied with oxygenated blood, is 

 awakened as soon as the connection with the uterine sinuses 

 is interrupted. As soon as the supply of CO 2 rises to a certain 

 point, an impulse of inspiration is generated, and as the infant 

 breathes the lungs assume a condition of partial expansion. 

 With diminished resistance in the expanded lungs the amount of 

 blood in the pulmonary circulation increases, and as the amount 

 passing through the ductus arteriosus consequently decreases, 

 this soon is obliterated. At the same time the amount of blood 

 returning to the left auricle increases in quantity, and the intra- 

 auricular pressure becomes greater; then, too, the inferior vena 

 cava sends less blood, for the ductus venosus no longer carries 

 the blood from the placental circulation, and, therefore, the fora- 

 men ovale is not used, and is soon closed by the adhesion of its 

 valve-like curtain. Thus, the adult circulation is established 



