256 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



Aorta 



Closure of the Foramen Ovale. The free edge of the septum I is, in 

 embryos of 10 to 15 mm., directed dorsad and cephalad (Fig. 261 C). 

 Gradually, in later stages (Figs. 264 and 265), its caudal and dorsal pro- 

 longation grows cephalad and ventrad until its free edge is so directed. 

 Coincident with this change, the septum II, with its free edge directed at 

 first ventrad and cauded, shifts until its free edge is directed dorsad and 

 cephalad, and overlaps the septum I (Figs. 261 0,264 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 limbus of the' previously fused septum II 

 and left sinus valve, and unites with it. The depression formed by the 

 thinner walled septum I is the fossa ovalis. : 



The Pulmonary Veins. In embryos of 6 to 7 mm., a single vein 

 (arising in the cat from a peripulmonary plexus, Brown, 1913) drains into 



the caudal wall of the left atrium 

 at the left of the septum I (Fig. 

 261 C). This vein bifurcates 

 into right and left pulmonary 

 veins which divide again before 

 entering the lungs. As the 

 atrium grows, the proximal por- 

 tion of the pulmonary vein is 

 taken up into the atrial wall. 

 As a result, at first two, then 

 four pulmonary veins open into 

 the left atrium. 



Origin of the Aorta and 

 Pulmonary Artery. In embryos 



of 4 to 6 mm. there arise in the aortic bulb (including its distal truncus 

 arteriosus) longitudinal thickenings, four in the distal half, two in the 

 proximal half. Of the four distal thickenings (Fig. 266), two, which may 

 be designated a and c, are larger than the other thickenings, b and d. 

 Thickenings a and c, which distally occupy left and right positions in 

 the bulb, meet, fuse, and divide the bulb into a dorsally placed aorta 

 and ventrally placed pulmonary trunk (Fig. 267). Traced proximally 

 they pursue a clockwise, spiral course, a shifting from left to ventral, and 

 c from right to dorsal, both becoming continuous with the proximal 

 swellings. Thickenings b and d are also prominent at one point 



Pulmonary artery 



FIG. 266. Scheme showing division of 

 bulbus cordis and its thickening into aorta 

 and pulmonary artery with their valves. 

 (Explanation in text.) 



