THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



247 



alter the relations of the vessels. The heart migrates from its original position 

 in the cervical region into the thorax, and this produces an elongation of the 

 carotid arteries and an apparent shortening of the arch of the aorta; conse- 



Vert. <Lrt. 



-Sub. inter- 

 Co it at art. 



FIG. 222. 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. 



quently the subclavian artery on the left side arises relatively nearer the heart. 



The arteries of the brain arise as branches of the internal carotid and circu- 



lus arteriosus. The anterior cerebral artery and the middle cerebral artery arise 



Circulus arteriosus 



Middle cerebral 

 artery 



Ant. cerebral 

 artery 



Basilar artery 

 Int. carotid artery 



FIG. 223. Brain and arteries of a human embryo of 9 mm. Mall. 



primarily from a common stem which in turn is a branch of the most cranial 

 part of the internal carotid (Figs. 223 and 224). The posterior cerebral artery 

 arises as a branch of the circulus arteriosus (Fig. 224). 



