EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 25S 



264 and 265). The opening between these septa persists until after birth as the 

 foramen ovale. 



During fetal life the left atrium receives little blood from the lungs, so that 

 the pressure is much greater in the right atrium. As a result, the septum I is 

 pushed to the left and the blood flows from the right into the left atrium through 

 the foramen ovale. After birth, the left atrium receives from the expanding lungs 

 as much blood as the right atrium, the septum I is pressed against the Umbus of 



Crista tcrmiiialis 

 Sept. 11+ L. valve of 

 sinus ioioiiis 

 Septum I 



Sup. vena cava (opened) 



Inf. vena cava 

 Valve of inf. vena ca 



Valve of coronary sinus- 



Tricuspid valve 



■Foramen ovale 



Aorta 



.Semilunar valves 

 ^ of pulmonary artery 



R. ventricle 



Fig. 263. — Lateral dissection of the heart of a 105 mm. human fetus viewed from the right side. X 7. 



septum II, and soon fuses with it. The depression formed by the thinner walled 

 septum I is the fossa ovalis. 



The foramen ovale may fail to close soon after birth and the mixed blood produces a 

 purplish hue in the child which is known popularly as a "blue baby." This condition may 

 be persistent in adult life. Incomplete closure occurs in about one in four cases, but actual 

 mingling of the blood is rare, due to an approximation of the overlapping septal folds during 

 atrial systole. 



Pulmonary Veins. — In embryos of 6 to 7 mm. a single vein (arising in the 

 cat from a peripulmonary plexus. Brown, Anat. Rec, vol. 7, 1913) drains into 



