THE CHANGES IN THE CIRCULATION AT BIRTH. 587 



longer reach the aorta, but passes entirely along the pulmonary 

 arteries to the lungs. From the lungs it is returned by the 

 pulmonary veins, which are now greatly enlarged, to the left 

 auricle, and so to the left ventricle, which drives it not only to 

 the head and upper limbs, but also along the dorsal aorta to the 

 hinder part of the body. 



By obliteration of the ductus venosus, all the blood in the 

 hepatic portal vein is compelled to pass through the capillaries 

 of the liver in order to reach the posterior vena cava. In other 

 words, by these three changes — obliteration of the ductus arte- 

 riosus, obliteration of the ductus venosus, and closure of the 

 foramen ovale — the foetal circulation has been converted into that 

 of the adult. 



These changes do not occur immediately on birth, nor are 

 they effected simultaneously. 



Obliteration of the allantoic or hypogastric arteries occurs 

 first ; it is effected partly by contraction of the entire vessels, 

 but chiefly by thickening of their inner coats, and is usually 

 completed by the third or fourth day after birth. 



The allantoic veins and the ductus venosus remain open 

 rather longer, but are generally obliterated by the sixth or 

 seventh day. 



The ductus arteriosus, according to Allen Thomson, ' is rarely 

 found open after the eighth or tenth day, and by three weeks it 

 has, in almost all instances, become completely impervious.' 



Closure of the foramen ovale is the last of the changes to be 

 completed. The closure is at first effected merely by the valve, 

 which projects into the left auricle, being kept closely applied 

 to the margin of the aperture by pressure of the increased 

 quantity of blood now returning by the pulmonary veins. At a 

 later stage the edge of the valve gradually coalesces with the 

 margin of the opening, but the union often remains incomplete 

 for some months ; and it not unfrequently happens that an 

 oblique valvular aperture, large enough to admit a probe, 

 persists for the first year of infancy, and may even be perma- 

 nent throughout life, in which case a direct passage of venous 

 blood into the left auricle is liable to occur, especially on over- 

 exertion. 



