THE HEART. 473 



before entering the inferior cava ; hence the large size of this organ, especially at 

 an early period of foetal life. 



3. That the right auricle is the point of meeting of a double current, the 

 blood in the inferior cava being guided by the Eustachian valve into the left 

 auricle, whilst that in the superior cava descends into the right ventricle. At an 

 early period of foetal life it is highly probable that the two streams are quite dis- 

 tinct, 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 auricles 

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

 must take place. 



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

 vein, mixed with the blood from the portal vein and 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 upper 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 by about the tenth 

 day after birth ; the valvular fold above mentioned becomes adherent to the 

 margins of the foramen for the greater part of its circumference, but above 

 a slit-like opening is left between the two auricles which sometimes remains per- 

 sistent. 



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 ar> impervious cord which serves to connect the left pulmonary 

 artery to the descending 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 superior vesical artery, and the part extending from the side of the bladder 

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

 and projects as a fibrous cord toward the abdominal cavity, carrying on it a fold 

 of peritoneum and separating two of the fossae of the peritoneum spoken of in the 

 section on the surgical anatomy of direct inguinal hernia. 



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. 



