THE DEVELOPMENT OF THE VASCULAR SYSTEM 243 



on the right side arise by a common stem, the innominate artery, which in 

 turn is a branch of the arch of the aorta. On the left side, for the same 

 reason, the common carotid is a branch of the arch of the aorta. The fifth 

 aortic arch from the beginning is rudimentary and disappears very early. 

 The sixth arch on each side undergoes wide changes. A branch from each 

 enters the corresponding lung. On the right side the portion of the sixth 

 arch between the branch which enters the lung and the dorsal aortic root 

 disappears, as does also that portion of the right dorsal aortic root between 

 the subclavian artery and the original bifurcation of the dorsal aorta. On 

 the left side, however, that portion of the sixth arch between the branch 

 which enters the lung and the dorsal aortic root persists until birth as the 

 ductus arleriosiis (Botalli). This conveys the blood from the right ventricle 

 to the aorta until the lungs become functional (Fig. 216); it then atrophies 



Int. carotid artery 



Vertebral artery 



Segmental cervical artery 



^' r Pulmonary artery 



FIG. 221. Diagram cf the aortic arches (III, IV, VI) and segmented cervical arteries 

 cf a 10 mm. human embryo. His. 



and becomes the ligamentum arteriosum. In the meantime the septum 

 aorticum has divided the original ventral aortic trunk into two vessels (see 

 P- 2 35)j one of the vessels communicates with the left ventricle and is the 

 proximal part of the arch of the aorta, the other communicates with the right 

 ventricle and becomes the large pulmonary artery (Fig. 212). 



In human embryos of 10 mm. the dorsal aortic root on each side gives off 

 several lateral branches the segmental cervical vessels (Fig. 221). The 

 first of these (first cervical, suboccipital), which arises nearly opposite the 

 fourth aortic arch, is a companion, as it were, to the hypoglossal nerve, and 

 sends a branch cranially which unites with its fellow of the opposite side in- 

 side the skull to form the basilar artery. The basilar artery again bifurcates 

 and each branch unites with the corresponding internal carotid by means of 

 the circulus arteriosus (Fig. 223). The other segmental cervical vessels 

 arise from the aortic root at intervals, the eighth arising near the point of 



