880 THE VASCULAR SYSTEM: 
thickenings which meet and fuse together from their distal to their proximal ends, 
forming a septum which divides the interior of the aortic bulb into two parts, and then 
projects downwards into the ventricular chamber till it meets and fuses with the anterior 
part of the upper border of the ventricular septum. The upper or distal part of the 
septum of the aortic bulb commences from the dorsal wall of the cavity between the 
fourth and fifth cephalic arches, and it is placed transversely, but its lower end lies 
more antero-posteriorly, therefore it twists spirally as it descends, and as a result the right 
part of the ventricle is thrown into continuity with the fifth cephalic aortic arches, whilst 
the left part of the ventricle retains continuity with the remaining cephalic aortic arches. — 
After the septum of the aortic bulb is completed, grooves appear along its margins on the 
surface of the bulb; the grooves deepen until they divide the septum and consequently 
the bulb into two parts; the part in connexion with the right ventricle and the fifth 
aortic arch becomes the pulmonary artery, and the part in connexion with the left 
ventricle and the remaining arches becomes the ascending aorta. 
The see of the primitive auricle into right and left portions is indicated 
externally by the appearance of a groove on the upper and posterior wall ; opposite this 
Tth segmental artery Dorsal aorta Vertebral artery Basilar artery 
Posterior 
SSS, WA artery 
Aorta Subclavian 
artery 
Posterior con 
municating 
artery 
Internal j 
carotid artery — 
9 
! 
Pulmonary artery ~ 
Ist cephalic 
aortic arch 
2nd cephalic aortic arch 
3rd cephalic aortic arch 
4th cephalic aortic arch 
Superior vena 
caval blood- 
stream ( 
Foramen ovale 5th cephalic aortic arch ne Middle | 
cerebral artery 
Septum dividing the aortic bulb 
Inferior vena Internal carotid artery Anterior cerebral 
eaval blood- 
artery} 
stream i y 
Inter-auricular/ 
septum Inter-ventricular septum 
Fig. 601.—DEVELOPMENT OF THE HEART AND THE MAIN ARTERIES. 
Diagram of the heart, showing the formation of its septa, and of the cephalic portion of the arterial system. 
groove an auricular septum grows downwards in the interior of the auricle. Its lower 
border gradually approaches the endocardial cushion in the auricular canal, and for a time 
a small opening is left between the upper ends of the fused endocardial cushions and the 
lower edge of the septum. This is the ostium primum ; it is closed by the fusion of the 
septum with the endocardial cushions, but before its closure is completed an aperture 
appears in the upper part of the septum; this latter aperture, the ostium secundum, 
becomes the foramen ovale. <A second auricular septum, the septum secundum, grows down- 
wards to the right of the first septum ; its lower margin grows downwards past the foramen 
ovale, but stops some distance from the posterior wall of the auricle, and this margin 
forms the limbus Vieussenii, which bounds a fossa, the fossa ovalis, in front and below, 
whilst the floor of the fossa is formed by the primary septum, and the foramen which 
lies at the upper part of the fossa is closed after birth by the fusion of the primary and 
secondary septa. 
The sinus venosus, which in the early stages receives the vitelline veins from the 
yolk sac, the allantoic or umbilical veins froma the placenta, and the ducts of Cuvier 
which return the blood from the Wolffian bodies and the body of the embryo, is the only 
portion of the primitive heart which is not divided into two parts by the formation of a 
septum. It lies at first below and behind the auricle, with which it communicates freely, 
