1030 THE VASCULAK SYSTEM. 



The ventral branch of the seventh cervical segmental artery on each side forms the trunk of 

 the subclavian artery, from the origin of the vertebral to the origin of the internal mammary, 

 and that part of the internal mammary which extends from its origin to the upper margin of 

 the first costal cartilage. The remainder of the internal mammary artery, and the superior and 

 inferior epigastric arteries, are derived from a longitudinal anastomosis which forms between the 

 terminal extremities of the ventral branches of the intersegmental arteries. 



The lateral ramus of the ventral branch of the seventh cervical intersegmental artery forms 

 the trunk of the subclavian artery beyond the origin of the internal mammary branch, and from 

 its prolongation into the upper extremity are formed the main arterial stems of the upper limb. 



The superior intercostal arteries are derived from longitudinal anastomoses which connect 

 together the ventral branches of the intersegmental arteries, on the inner sides of the dorsal 

 parts of the ribs. 



The anterior and posterior spinal arteries are derived from plexiform anastomoses which form 

 on the dorsal and ventral aspects of the neural tube between the terminal ends of the dorsal and 

 ventral neural branches of the neural rami of the posterior divisions of the intersegmental arteries. 



The lateral branches of the primitive dorsal aortse supply structures derived from the inter- 

 mediate cell tract, and from them are formed the renal, the suprarenal, the inferior phrenic, and 

 the internal spermatic and ovarian arteries. 



The ventral branches of the primitive dorsal aortse are not definitely segmental or inter- 

 segmental in arrangement. In the early stages they pass not only to the gut wall but also, 

 beyond it, to the wall of the yolk-sac. They are connected together by longitudinal anastomosing 

 channels which lie in the dorsal mesentery of the gut and also upon the wall of the gut itself. 

 As the yolk-sac atrophies the prolongations of the ventral branches which pass to its walls 

 disappear and, simultaneously, the portions of the corresponding vessels of opposite sides, which 

 lie in the mesentery, dorsal to the gut, and the longitudinal anastomoses which connect them, 

 fuse together to form unpaired stem-trunks from which the three great vessels of the abdominal 

 part of the alimentary canal are derived, namely, the cceliac, the superior mesenteric, and the 

 inferior mesenteric arteries ; but the original stem of each of these three important vessels is not 

 that which eventually forms its origin from the abdominal part of the aorta, for the coeliac 

 artery, which originally arose opposite the seventh cervical segment, wanders caudalwards to the 

 twelfth thoracic segment as the roots of origin of the ventral vessels which are situated nearer 

 the head disappear ; and in the same manner the superior mesenteric is transposed from the 

 level of the second thoracic to the level of the first lumbar segment, and the inferior mesenteric 

 wanders from the twelfth thoracic to the third lumbar segment. 



The Umbilical and Iliac Arteries. It was pointed out, in the account of general features of 

 embryology, that the umbilical arteries which carry blood to the placental area of the chorion 

 arise, in a human embryo about 1*38 mm. long, about the level where the fourth cervical 

 mesodermal somite will be developed at a later stage. They spring from plexuses formed, on 



the lateral walls of the caudal 



Aorta Aorta part of the primitive gut, by 



^^j|^^^^ the anastomosis of some of the 

 dfftj Hfei most caudally situated ventral 



. External iliac artery,_^^T\ Jvb&L T vitelline branches of the 



Pronephric duct - ^^^^^ ^f primitive dorsal aorta. The 



Secondary part of-- --m- Jp origins of the arteries are 



umbilical artery ^^ ^ gradually moved caudally as 



' Primary part of -5k f the J^T , S rOWS > U ? M1 ' 



umbilical artery qj fj eventually, they spring trom 



the primitive dorsal aorta, 



FIG. 825. DIAGRAM SHOWING THE FORMATION OF THE SECONDARY 



PART OF THE PRIMITIVE UMBILICAL ARTERY. 



lumbar segment. As each 



umbilical artery passes from 



its origin on the ventral wall of the primitive dorsal aorta to the body-stalk it lies to the medial 

 side of the pronephric duct. The ventral origin is, however, but temporary ; for, by the time the 

 embryo has attained a length of 5 mm., and the primitive dorsal aortas have fused to form the 

 permanent descending aorta, a new vessel has arisen, on each side, from the lateral part of the 

 caudal end of the aorta. This new vessel passes ventrally, to the lateral side of the Wolffian duct, 

 and then unites, on a plane ventral to the aorta, with the primitive umbilical artery of the same 

 side. After the union has taken place the ventral origin of the umbilical artery disappears, and 

 the primitive umbilical artery then arises from the side of the caudal end of the aorta. From 

 the newly formed vessel, which now constitutes the only origin of the umbilical artery, the 

 inferior glutseal artery, which is the primitive main artery of the lower limb, arises. At a later 

 period, and at a more dorsal level, a second branch arises from the dorsal root of origin of the 

 umbilical artery ; this is the second main vessel of the lower extremity, which becomes the 

 external iliac and the femoral arteries of the adult. As soon as the external iliac artery is formed 

 that portion of the umbilical stem which lies dorsal to it becomes the common iliac artery, and 

 the more ventral part, which descends into the pelvis minor, becomes the hypogastric artery. 

 But that portion of the original umbilical artery which runs along the side of the pelvis minor to 

 the ventral wall of the abdomen, then cephalwards to the umbilicus and through the umbilicus 

 to the placenta, is still called the umbilical artery. After birth, when the placental circulation 

 ceases, the greater part of the intra-abdominal portion of the umbilical artery atrophies and 



