THE VASCULAR SYSTEM 



377 



arteries become reduced in number to a single vessel formed by the 

 coalescence of a pair of arteries. This vessel persists as the superior 

 mesenteric artery of the adult. In a 2.6 mm. embryo, there are two pairs 

 of aortic arches, and by the time the embryo is 7 mm. long, five pairs of 

 aortic arches are present. A series of twenty-seven branches of the 

 dorsal aorta form the intersegmental arteries of the body-wall. Celiac, 

 superior mesenteric, and inferior mesenteric arteries develop from the 

 vitelline arteries. Three types of branches develop from the dorsal aorta, 

 somatic or intersegmental, lateral or intermediate, and ventral or visceral 

 branches. Lateral or intermediate branches are limited to the trunk 

 region. From the dorsal branches arise the metameric vertebral, costal, 

 and lumbar arteries. The vertebral arteries below the brain combine as 

 the basilar artery. The subclavian arteries develop from the seventh 

 cervical intersegmental arteries of the embryo. 



Fig. 316. — Three stages in the development of the arteries of the fore limb of the 

 white mouse. A, 8 days; B; 9 days; C, 10 days; a, aorta; b, brachial plexus. (The 

 vessels are extremely variable, not agreeing even on the two sides of a single individual.) 

 (From Kingsley's " Comparative Anatomy of Vertebrates," after Goppert.) 



From the intermediate branches of the dorsal aorta come the renal, 

 adrenal, phrenic, and spermatic or ovarian arteries. The vessels which 

 supply the intestine, the celiac and mesenteric, are derived from the 

 ventral visceral arteries. The aortic arches, limited to the head region, 

 are also visceral. The umbilical arteries during ontogenesis change greatly 

 in their relations. They arise in a 1.5 mm. embryo opposite the fourth 

 cervical myotome. In a 5 mm. embryo, they have been shifted backward 

 to a connexion opposite the fourth lumbar vertebra. In the adult," they 

 persist as the iliac arteries. The median sacral artery is the continuation 

 of the dorsal aorta into the sacrum and coccyx. (Fig. 315) 



The arteries of the upper and lower extremities grow into them from 

 the dorsal aorta. The brachial artery of the arm is formed by the union 

 of three segmental vessels, the middle of which belongs to the last cervical 

 segment. Extending into the forearm, this artery becomes the interos- 

 seous artery. In the hand region five branches are given off to the five 

 fingers. Through the growth of a median artery paralleling the interosse- 

 ous artery, the latter loses its connexion with the digital arteries. The 

 ulnar artery is added as a third vessel in the forearm, and in the hand 



