% THE SKELETON 359 



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 deoxygenation 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 impervious fibrous cord between the third and tenth days 

 after birth. 



The hypogastric arteries, umbilical vein and ductus venosus 

 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 

 protovertebrae, or somites, has been observed. The notochord 

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

 of the vertebra are developed, though it does not itself become 

 those bodies. The protovertebrae were seen to lie longitudinally 

 on either side of the notochord. These grow around the neural 

 canal dorsally and the notochord ventrally to form the vertebra?. 



