664 REPRODUCTIVE ORGANS. 



fluence of exposure to the air, shows that this is not the essential, 

 but only a contributing, cause. It is the stoppage of the placenta! 

 circulation which starts the respiratory movements. 



Although during fetal life some blood flows through the pulmo- 

 nary capillaries, still the amount is small, and, there being no air 

 in the pulmonary alveoli, the lungs will sink if placed in water. 

 The first respiratory movement causes an enlargement of the 

 thoracic cavity and a consequent distention of the lungs, the air 

 passing h to the alveoli, and the blood, which is at the time 

 in the pulmonary capillaries, becomes oxygenated and returns to 

 the left auricle as arterial blood. The expansion of the thorax 

 reduces the resistance to the flow of the blood through the pulmo- 

 nary circulation, and as a result a large amount of blood goes to 

 the lungs ; this means a lessened amount through the ductus 

 arteriosus, and, following the law that a diminution of function is 

 followed by atrophy, this vessel begins to diminish in size, and 

 becomes closed between the fourth and tenth days, and in later 

 life is to be found as a fibrous cord between the left pulmonary 

 artery and the aorta. 



With the termination of the placental circulation the flow 

 through the ductus venosus ceases, and within a few days this 

 vessel closes, and remains only as a fibrous cord in the fissure of 

 the same name in the liver : that portion of the umbilical vein 

 which is within the body of the child becomes the round ligament 

 of the liver. The blood flowing into the right auricle from the 

 inferior vena cava finds it easier to pass into the right ventricle 

 than into the left auricle, which is now filled with blood from the 

 lungs, and hence takes this course, while the blood cannot flow 

 into the right auricle through the foramen ovale by reason of the 

 valve which has been forming in the left auricle during the latter 

 part of intra-uterine life to close this opening. The opening is 

 not permanently closed for a considerable time after birth, in some 

 cases a year, and sometimes not at all. As a result of these 

 various changes the fetal circulation becomes converted into that 

 of the adult. 



