Chap, ii.] PREGNANCY AND BIRTH. 1135 



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 in- 

 stead 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 be- 

 comes obliterated. The obliteration is ultimately secured by 

 proliferation of the internal coat, in which even before birth 

 the sub-epithelial layer is unusually developed, a thrombus 

 (§ 33) at times helping, but before this takes place the closure 

 seems to be assisted by the mechanical arrangements of the 

 parts. Corresponding to the greater flow into the pulmonary 

 artery, a larger and larger quantity of blood returns from the 

 pulmonary veins into the 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 rinding little resistance in the 

 direction of the ventricle, which now readily discharges its con- 

 tents 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 foramen 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 sepa- 

 ration 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. 



