702 THE THORAX. 



quantity from the left ventricle, is distributed to the lower extremities; hence the 

 small size and imperfect development of these parts at birth. 



CHANGES IN THE VASCULAR SYSTEM AT BIRTH. 



At birth, when respiration is established, an increased amount of blood from 

 the pulmonary artery passes through the lungs, which now perform their office as 

 respiratory organs, and, at the same time, the placental circulation is cut off. 

 The foramen ovale becomes gradually closed by about the tenth day after birth, 

 a valvular fold rising up on the left side of its margin, and ultimately above its 

 upper part ; this valve becomes adherent to the margins of the foramen for the 

 greater part of its circumference, but, above, a valvular opening is left between the 

 two auricles, which sometimes remains persistent. 



The ductus arteriosus begins to contract immediately after respiration is estab- 

 lished, becomes completely closed from the fourth to the tenth day, and ultimately 

 degenerates into an impervious cord, which serves to connect the left pulmonary 

 artery to the concavity of the arch of the aorta. 



Of the umbilical or hypogastric arteries, the portion continued on to the bladder 

 from the trunk of the corresponding internal iliac remains pervious, as the supe- 

 rior vesical artery ; and the part between the fundus of the bladder and the 

 umbilicus becomes obliterated between the second and fifth days after birth, and 

 forms the anterior true ligament of this viscus. 



The umbilical vein and ductus venosus become completely obliterated between 

 the second and fifth days after birth, and ultimately dwindle to fibrous cords ; the 

 former becoming the round ligament of the liver, the latfer the fibrous cord, 

 which, in the adult, may be traced along the fissure of the ductus venosus. 



