680 F(ETAL CIRCULATION. [BOOK iv. 



returns to the left auricle brighter and richer in oxygen than ever 

 was the foetal blood before. With the diminution of resistance in 

 the pulmonary circulation caused by the expansion of the thorax, 

 a larger supply of blood passes into the pulmonary artery instead 

 of into the ductus arteriosus, and this derivation of the contents 

 of the right ventricle increasing with the continued respiratory 

 movements, the current through the latter canal at last ceases 

 altogether, and its channel shortly after birth becomes obliterated. 

 Corresponding to the greater flow into the pulmonary artery, a 

 larger and larger quantity of blood returns from the pulmonary 

 veins into tiie left auricle. At the same time the current through 

 the ductus venosus from the umbilical vein having ceased, the flow 

 from the inferior cava has diminished ; and the blood of the right 

 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 over-burdened 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. 



