246 



TEXT-BOOK OF EMBRYOLOGY. 



arteriosus (Botalli). This conveys the blood from the right ventricle to the 

 aorta until the lungs become functional (Fig. 207); it then atrophies and be- 

 comes the ligamentum arleriosum. In the meantime the septum aorticum has 

 divided the original ventral aortic trunk into two vessels (see p. 230) ; 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 be- 

 comes the large pulmonary artery (Fig. 203). 



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 inside the skull 

 to form the basilar artery. The basilar artery again bifurcates and each branch 



Int. carotid artery 



Vertebral artery 



* '- Segmental cervical artery 



Pulmonary artery 



Fig. 221. — Diagram of the aortic arches (III, IV, VI) and segmental cervical arteries 

 of a 10 mm. human embryo. His. 



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 bifurcation of the aorta. In a 

 short time a longitudinal anastomosis appears between these segmental arteries, 

 which extends as far as the seventh (Fig. 222). The proximal ends of the first 

 six disappear, and the longitudinal vessel forms the vertebral artery which then 

 opens into the aortic root through the seventh segmental artery, and which is 

 continued cranially as the basilar artery (Fig. 223). The seventh (it is held by 

 some to be the sixth) segmental artery becomes the subclavian, and conse- 

 quently the vertebral opens into the subclavian, as in the adult (Fig. 222). But 

 it should be borne in mind that the right subclavian artery is more than equiva- 

 lent to the left, since the proximal part of the former is made up of the fourth 

 aortic arch and a part of the aortic root (see Figs. 219 and 220). Further- 

 more, changes occur in the position of the heart during development, which 



