768 THE THORAX. 



an early period of foetal life, it is highly probable that the two streams are quite 

 distinct; for the inferior cava opens almost directly into the left auricle, and 

 the Eustachian valve would exclude the current along the vein from entering 

 the right ventricle. At a later period, as the separation between the two auri- 

 cles becomes more distinct, it seems probable that some .mixture of the two 

 streams must take place. 



4. The blood carried from the placenta to the foetus by the umbilical vein, 

 mixed with the blood from the inferior cava, passes almost directly to the arch 

 of the aorta, and is distributed by the branches of that vessel to the head and 

 upper extremities : hence the large size and perfect development of those parts 

 at birth. 



5. The blood contained in the descending aorta, chiefly derived from that 

 which has already circulated through the head and limbs, together with a small 

 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-in by about the tenth day after 

 birth, a valvular fold rises up on the left side of its margin, and ultimately 

 above its upper part ; this valve becomes adherent to the margins of the fora- 

 men 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 ulti- 

 mately 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 the bladder. 



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 latter, the fibrous cord, 

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



