394 THE NUTRITION OF THE EMBRYO. [BOOK iv. 



auricle finding little resistance in the direction of the ventricle, 

 which now readily discharges its contents into the pulmonary 

 artery, but finding in the left auricle, which is continually being 

 filled from the lungs, an obstacle to its passage through the fora- 

 men ovale, ceases to take that course. Any return of blood from 

 the now vigorous and active left auricle into the right auricle 

 is prevented by the valve which, during the latter stages of intra- 

 uterine life, has been growing up in the left auricle over the 

 foramen ovale. At birth the edge of this valve is to a certain 

 extent free so that, in case of an emergency, as when the pulmonary 

 circulation is obstructed, a direct escape of blood into the left 

 auricle from the overburdened right auricle can take place. 

 Eventually, in the course of the first year, adhesion takes place, 

 and the separation of the two auricles becomes complete. With 

 its larger supply of blood and greater work the left ventricle 

 acquires the greater thickness characteristic of it during life. Thus 

 the foetal circulation, in consequence of the respiratory movements 

 to which its interruption gives rise, changes its course into that 

 characteristic of the adult. 



