DEVELOPMENT OF THE HEART. 685 



no longer admits a free access from the right auricle through the 

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

 more closely against the edges of the septum, becomes after a time 

 adherent throughout, and obliterates the opening altogether. The 

 cutting off of the placental circulation diminishes at the same time 

 the quantity of blood arriving at the heart by the inferior vena 

 cava. It is evident, indeed, that the same quantity of blood which 

 previously returned from the placenta by the inferior cava, on the 

 right side of the 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 fcetal circulation, represented in Fig. 272, is then replaced 

 by the adult circulation, represented in Fig. 273. 



Fig. 273. 



Diagram of ADULT CiscrLATroxTHRotraHTHK HEART. ", n. Superior and inferior vena; 

 cavse. >>. Rijrht ventricle, c. Pulmonary artery, dividing into right and left branches, d. Pulmo- 

 nary vein. e. Left ventricle. /. Aorta. 



That portion of the septum of the auricles, originally occupied 

 by the foramen ovale, is accordingly constituted, in the adult con- 

 dition, by the valve of the foramen ovale, which has become adhe- 



