702 REPRODUCTION. 



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. 



This change depends on the commencement of respiration, by which 

 the quantity of blood passing through the lungs is largely increased. 

 The left auricle, thus supplied to its full capacity with blood returning 

 from the lungs, no longer admits the entrance of a further quantity 

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

 backward against the septum, becomes after a time adherent through, 

 out, and obliterates the opening. The cutting off of the placental cir- 

 culation also diminishes the volume of blood in the inferior vena cava. 

 It is evident that the same quantity which previously returned from 



FIG. 252. 



DIAGRAM OF THE ADULT CIRCULATION THROUGH THE HEART. a, a. Superior and inferior vense 

 cavse. 6. Eight ventricle, c. Pulmonary artery, dividing into right and left branches, d. Pul- 

 monary vein. e. Left ventricle. /. Aorta. 



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, 

 on the left side of the same septum ; and, the pressure being thus 

 equalized in the right and left auricles, there is no mixture of the blood 

 between the two. 



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

 sented in Fig. 252. 



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

 foramen ovale, is accordingly formed, after birth, by the valve of this 

 foramen, which has become adherent to its edges. The septum in the 

 adult heart is thinner at this spot than elsewhere ; and presents, on its 



