CHANGES IN THE CIRCULATION AT BIRTH. 663 



discharges its blood into the right auricle of the heart, where, 

 guided by the Eustachian valve, it is directed into the left auricle 

 through the foramen ovale. From this cavity it passes into the 

 left ventricle, thence into the aorta, which distributes it to the 

 head and upper extremities. It will be seen from this description 

 that to these three portions of the body the blood from the 

 placenta is distributed. This blood is not very pure, for it is 

 deteriorated by admixture with the impure blood returning from 

 the lower extremities, with which it mingles in the ascending 

 vena cava ; but it is the purest and most nutritious blood the 

 fetus receives, and this accounts for the greater development of 

 the upper portion of the body as compared with the lower, which 

 is so striking a feature in the newborn babe. 



The blood returns from the head and upper extremities through 

 the descending vena cava to the right auricle, and thence passes 

 into the right ventricle. There is probably always a slight mixing 

 of the currents in the right auricle, that returning from the 

 placenta and that from the descending vena cava, but at first this 

 is very slight ; later, it is doubtless greater. From the right 

 ventricle the blood passes into the pulmonary artery, a very small 

 portion going through the capillaries of the lungs, the larger part 

 passing through the ductus arteriosus into the aorta, passing down 

 this vessel to the common and internal iliacs, from which latter 

 are given off the hypogastric or umbilical arteries by which the 

 blood is conveyed to the placenta. 



By comparing this description with that of the circulation in 

 the adult the points of difference will be seen. It may be well 

 to note here that there are six principal points of difference be- 

 tween the fetal and the adult circulatory apparatus, besides less 

 important ones of size and shape. These points of difference are 

 the presence in the fetal heart of the Eustachian valve and the 

 foramen ovale, in the venous system of the umbilical vein and the 

 ductus venosus, and in the arterial system of the umbilical arteries 

 and the ductus arteriosus. 



Changes in the Circulation at Birth. During intra- 

 uterine life the respiratory center in the medulla is supplied with 

 blood containing sufficient oxygen to prevent any inspiratory im- 

 pulse, and there is therefore during this period no attempt at 

 respiration on the part of the fetus. As soon, however, as the 

 connection between the parent and the child is severed, whether 

 by separation of the placenta or by tying of the umbilical cord, 

 the respiratory center, being no longer supplied with oxygen, sends 

 out impulses to the respiratory muscles, and respiration begins. 

 This may be hastened or assisted by slapping the skin or dashing 

 water upon it, but under ordinary circumstances these measures 

 are not called for. The fact that respiration will take place while 

 the fetus is still enclosed in its membranes, without the reflex in- 



