360 REPRODUCTION 



umbilical vein are the organs which distinguish the placental 

 circulation, and they all partially disappear after birth, as 

 will be immediately seen. 



3. Adult Circulation. The circulation as it exists in the 

 adult has been described. It is only necessary to see what 

 changes mark its establishment. 



When the child is born detachment of the placenta, or 

 ligation of the cord, stops the placental circulation. The 

 first noticeable effect comes from the consequent deoxygen- 

 ation of the blood. The respiratory center is stimulated and 

 the child gasps to fill the hitherto collapsed lungs with air. 

 Owing to the diminished resistance in the expanded lungs, 

 the pulmonary artery begins to carry most of the blood from 

 the right ventricle, and the ductus arteriosus commences to 

 atrophy. .Before birth, too, the Eustachian valve becomes 

 less distinct and the foramen ovale partly closes. At labor 

 a kind of valve guards the opening of the foramen ovale and 

 allows the escape possibly of a little blood from the right 

 into the left auricle, but none in the opposite direction. It 

 commonly closes about the tenth day of extra-uterine life. 

 The ductus arteriosus is reduced to the condition of an im- 

 pervious fibrous cord between the third and tenth days after 

 birth. 



The hypogastric arteries, umbilical vein and ductus ven- 

 osus are closed between the second and fourth days. That 

 part of each hypogastric artery between the internal iliac 

 and the upper lateral part of the bladder remains in adult life 

 as the superior vesical artery; the part between this point 

 and the umbilicus is that which atrophies. The umbilical 

 vein remains as the round ligament of the liver. The ductus 

 venosus is represented by a fibrous cord in the fissure for 

 the ductus venosus in the liver. 



The Skeleton. The appearance of the notochord and of 

 the protovertebrse, or somites, has been observed. The noto- 

 chord becomes a thin line of soft cartilage, around which the 

 bodies of the vertebra are developed, though it does not 



