DEVELOPMENT OF THE VASCULAR SYSTEM. 807 



forming the valve, do not elongate in proportion. The valve, accord- 

 ing^, becomes drawn downward more closely toward the foramen ovale. 

 It thus comes in contact with the edges of the inter-auricular septum, 

 and unites with its substance ; the adhesion taking place first at the 

 lower and posterior portion, and proceeding gradually upward and for- 

 ward, so that the passage from the right auricle to the left becomes con- 

 stantly more oblique in direction. 



At the same time there is an alteration in the position of the inferior 

 vena cava. This vessel, which at first looked transversely toward the 

 foramen ovale, becomes directed more obliquely forward ; and thus, the 

 Eustachian valve having nearly disappeared, a part of the blood of the 

 inferior vena cava enters the right auricle, while the remainder still 

 passes through the equally oblique opening of the foramen ovale. 



At birth a change takes place, by which the foramen ovale is com- 

 pletely occluded, and all the blood coming through the inferior vena 

 cava is turned into the right auricle. 



The change depends upon the commencement of respiration. When 

 this occurs, a much larger quantity of blood is sent to the lungs, and 

 of course returns from them to the left auricle. The left auricle, 

 being thus filled with blood from the lungs, no longer admits the 

 entrance of a further quantity from the right auricle through the fora- 

 men ovale ; and the valve of the foramen, pressed backward against the 

 edges of the septum, becomes after a time adherent throughout, and 

 thus obliterates the opening. The cutting off of the placental circula- 

 tion diminishes at the same time the quantity of blood arriving at the 

 heart by the inferior vena cava. It is evident that the same quantity 

 of blood which previously returned from the placenta by the inferior 

 vena cava on the right side of the inter-auricular septum, now returns 

 from the lungs, by the pulmonary veins, upon the left side of the same 

 septum ; and it is owing to all these circumstances combined, that, while 

 before birth a portion of the blood always passed from the right auricle 

 to the left through the foramen ovale, no such passage takes place after 

 birth, since the pressure is then equal on both sides of the auricular 

 septum. 



The foetal circulation is then replaced by the adult circulation, repre- 

 sented in Fig. 316. 



That portion of the inter-auricular septum, originally occupied by the 

 foramen ovale, is accordingly constituted, in the adult condition, by the 

 valve of the foramen ovale, which has become adherent to the edges of 

 the septum. The septum in the adult heart is, therefore, thinner at this 

 spot than elsewhere ; and presents, on the side of the right auricle, an 

 oval depression, termed the fossa ovalis, which indicates the site of the 

 original foramen ovale. The fossa ovalis is surrounded by a slightly 

 raised ring, the annulus ovalis, representing the curvilinear edge of the 

 original inter-auricular septum. 



The foramen ovale is sometimes completely obliterated within a few 

 days after birth, but often remains partially pervious for several weeks 



