266 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



and the anterior choriodal artery; all of these supply the brain. Caudalward many small 

 branches to the brain wall are given off, and, quite late in development (48 mm. C R), 

 they form a true posterior cerebral artery (Mall). 



The ventral rami of the dorsal intersegmental arteries become promi- 

 nent in the thoracic and lumbar regions and persist as the intercostal and 

 lumbar arteries, segmentally arranged in the adult. Longitudinal precos- 

 tal anastomoses (Fig. 274) constitute the costo-cervical and thyreo-cervical 

 trunks (Fig. 275). 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 

 internal mammary and the superior and 

 inferior epigastric arteries are formed by 

 longitudinal ventral anastomoses (Fig. 

 274) between the extremities of the ven- 

 tral rami from the thoracic and lumbar 

 intersegmental arteries, beginning with the 

 second or third thoracic (Fig. 277). \ 



2. The lateral (visceral) branches of 

 the descending aortae are not segmentally 

 arranged. They supply structures arising 

 from the nephrotome region (mesonephros, 

 sexual glands, metanephros.and suprarenal 

 glands). From them later arise the renal, 

 suprarenal, inferior phrenic, and internal 

 spermatic or ovarian arteries. 



3. The ventral (splanchnic} branches 

 are at first rather definitely interseg- 

 mental. Primitively they form the paired 

 vitelline arteries to the yolk sac (Figs. 

 268 to 270). Coincident with the de- 

 generation 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 coeliac artery, the superior mesenteric, and the inferior 

 mesenteric (Fig. 271). 



The primitive coeliac axis arises opposite the seventh intersegmental artery. To- 

 gether 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 growth of the dorsal and ventral walls of the aorta. Similarly, the 

 superior mesenteric artery is displaced caudad ten segments, the inferior mesenteric artery 

 three segments. 



FIG. 277. The development of the 

 internal mammary and deep epigastric 

 arteries in a human embryo of 13 mm. 

 (Mall in McMurrich). 



