274 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



The internal carotids, after giving off the ophthalmic arteries, give rise cranially to the 

 anterior cerebral artery, from which arise later the middle cerebral artery and the anterior chorioidal 

 artery, all of which supply the brain. Caudalward many small branches to the brain wall 

 are given off and quite late in development (48 mm. embryos) these form a true posterior 

 cerebral artery (Mall). 



The ventral branches of the dorsal inter segmental arteries become large in the 

 thoracic and lumbar regions, and persist as the intercostal and lumbar arteries, 

 segmentally arranged in the adult. The subclavian and a portion of the internal 

 mammary artery are derived from the ventral ramus of the seventh cervical seg- 

 mental artery. The remainder of the in- 

 ternal mammary and the superior and in- 

 ferior epigastric arteries are formed by longi- 

 tudinal anastomoses between the extremities 

 of the ventral rami from the thoracic and 

 lumbar intersegmental arteries, beginning 

 with the second or third thoracic (Fig. 269). 

 The superior intercostal arteries arise from 

 anastomoses which connect the lateral rami 

 of these same branches on what will later be 

 the inner surfaces of the dorsal portions of 

 the ribs. 



The lateral branches of the descending 

 aortae are not segmentally arranged. They sup- 

 ply structures arising from the nephrotome 

 region (mesonephros, sexual glands, meta- 

 nephros and suprarenal glands) . From them 

 later arise the renal, suprarenal, inferior phre- 

 nic and internal spermatic or ovarian arteries. 



The ventral arteries are not definitely segmental or intersegmental. Primi- 

 tively they form the paired vitelline arteries to the yolk-sac (Figs. 261 and 263). 

 Coincident with the degeneration of the yolk-sac the prolongations of the ventral 

 vessels to its walls disappear and the paired persisting arteries, passing in the 

 mesentery to the gut, fuse to form unpaired vessels from which three large 

 arteries are derived, the cceliac artery, the superior mesenteric and the inferior 

 mesenteric (Fig. 264). 



The primitive cceliac axis arises opposite the seventh intersegmental artery. Together 

 with the mesenteric arteries, it migrates caudalward until eventually its origin is opposite 

 the twelfth thoracic segment (Mall). This migration, according to Evans, is due to the unequal 



FIG. 269. The development of the 

 internal mammary and deep epigastric ar- 

 teries in an embryo of 13 mm. (Mall from 

 McMurrich's Human Body). 



