684 DEVELOPMENT OF THE CIRCULATORY APPARATUS. 



membranous ridge, which can exert no influence on the direction 

 of the current of blood passing by it. Thus, the cavity of the infe- 

 rior vena cava, at its upper extremity, ceases to be separated from 

 that of the right auricle ; and a passage of blood from one to the 

 other may, therefore, more readily take place. 



Thirdly, the foramen ovale becomes partially closed by a valve 

 which passes across its orifice from behind forward. This valve, 

 which begins to be formed at a very early period, is called the 

 valve of the foramen ovale. It consists of a thin, fibrous sheet, which 

 grows from the posterior surface of the auricular cavity, just to the 

 left of the foramen ovale, and projects into the left auricle, its free 

 edge presenting a thin crescentic border, and being attached, by its 

 two extremities, to the auricular septum upon the left side. This 

 valve does not at first interfere at all with the flow of blood from 

 right to left, since its edge hangs freely and loosely into the cavity 

 of the left auricle. It only opposes, therefore, during the early 

 periods, any accidental regurgitation from left to right. 



But as gestation advances, while the walls of the heart con- 

 tinue to enlarge, and its cavities to expand in every direction, the 

 fibrous bundles, forming the valve, do not elongate in proportion- 

 The valve, accordingly, becomes drawn downward more and more 

 toward the foramen ovale. It thus comes in contact with the edges 

 of the interauricular septum, and unites with its substance; the 

 adhesion taking place first at the lower and posterior portion, and 

 proceeding gradually upward and forward, so as to make the pas- 

 sage, from the right auricle to the left, more and more oblique in 

 direction. 



At the same time, an alteration takes place in the position of the 

 inferior vena cava. This vessel, which at first looked transversely 

 toward the foramen ovale, becomes directed more obliquely for- 

 ward ; so that, the Eustachian valve having mostly disappeared, a 

 part of the blood of the inferior vena cava enters the right auricle, 

 while the remainder still passes through the equally oblique open- 

 ing of the foramen ovale. 



At the period of birth a change takes place, by which the 

 foramen ovale is completely occluded, and all the blood coming 

 through the inferior vena cava is turned into the right auricle. 



This change depends upon the commencement of respiration. 

 A much larger quantity of blood than before is then sent to the 

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

 left auricle, being then completely filled with the pulmonary blood, 



