THE DEVELOPMENT OF THE VASCULAR SYSTEM 



245 



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 

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

 position in the cervical region into the thorax, and this produces an elonga- 

 tion of the carotid arteries and an apparent shortening of the arch of the 

 aorta; consequently 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 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). 



Post, cerebral vein 

 (sup. petrosal sinus) 



irculus arteriosus 

 Transverse sinus 



Basilar artery 

 Int. jugular vein 



Confluence of sinuses 



Inf. sagittal sinus 

 Sup. sagittal sinus 

 Post, cerebral artery 



Ant. cerebral artery 

 Int. carotid artery 



FIG. 224. Brain, arteries and veins of a human embryo of 23 mm. Mali 



From the point of its bifurcation to its caudal end the aorta gives off 

 paired, segmental branches which accompany the segmental nerves. The 

 last (eighth) cervical branch and the first two thoracic branches undergo 

 longitudinal anastomoses, similar to those between the first seven cervical, 

 to form the superior intercostal artery (A. intercostalis suprema) which opens 

 into the subclavian (Fig. 222). The other thoracic branches persist as the 

 intercostal arteries; the lumbar branches persist as the lumbar arteries. At 

 the same time anastomoses are formed between the distal ends of the inter- 

 costal and lumbar arteries in the ventro-lateral region of the body wall, 

 which give rise, on the one hand, to the internal mammary artery and, on 

 the other hand, to the inferior epigastric artery. Of these two the former 

 opens into the subclavian, the latter into the external iliac. By a further 

 anastomosis the distal ends of the internal mammary and inferior epigastric 

 are joined, thus forming a continuous vessel from the subclavian to the 

 external iliac (Fig. 225). It is interesting to note that while originally all 

 the lateral branches of the aorta are arranged segmentally, many of them 



