THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



213 



bifurcation of the aorta. In a short time a longitudinal anastomosis appears 

 between these segmental arteries, which extends as far as the seventh (Fig. 

 184). The proximal ends of the first six disappear, and the longitudinal 





FIG. 184. Diagram illustrating the formation of the vertebral and superior intercostal arteries. 

 The broken lines represent the portions of the original segmental vessels that disappear. 

 Modified from Hochstetter. 



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. 185). The seventh (it is held by some to be the sixth) 



Circulus arteriosus 





Middle cerebral 

 artery 



Basilar artery 

 Int. carotid artery 



FIG. 185. Brain and arteries of a human embrvo of o mm. M all. 



segmental artery becomes the subclavian, and consequently the vertebral 

 opens into the subclavian, as in the adult (Fig. 184). But it should be 

 borne in mind that the right subclavian artery is more than equivalent to 

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



