636 THE BLOOD-VASCULAR SYSTEM 



yil (subclavian) and with the anterior ramus of lumbar V (external iliac), forms the 

 internal mammary (with its superior epigastric branch) and the inferior (deep) epigastric 

 arteries of the adult. 



In the sacral region, the adult shows evidence of segmental vessels in branches of the middle 

 and lateral 'sacral arteries; the latter probably representing a precostal anastomosis. Whether 

 the parietal.branches'may be derived directly from segmental sources, or whether they are vessels 

 of new formation, has not been determined embryologically. The obturator would appear to 

 be segmentaljfor it contributes a branch to the mammary anastomosis which persists in the adult 

 (pubic branches of obturator and inferior epigastric). If the connecting branch with the inferior 

 epigastric is large, the obturator may lose its connection with the hypogastric, in which case the 

 latter is said to arise from the former, or from the external iliac. 



One of the most interesting of the longitudinal anastomoses in connection with the dorsal 

 segmentals is the primitively bilateral preneural anastomosis extending ventral to the spinal 

 cordjand connected, beyond the first spinal segment, with each internal carotid by means of the 

 right and left aa. cerebrales vertebrales. The hypoglossus artery (p. 635) having lost its con- 

 nection with the internal carotid, leaves the spinal ramus of cervical I (third part of the 

 subclavian) to take over the major share of the cerebral supply. A process of blending by anas- 



FiG. 508. — Diagram to Show the Development of the Arteries of the Trunk from the 

 Aortic Arches and Segmental Arteries. ' 



The arteries which persist are black; those which degenerate are in outHne; those newly formed 



are shaded. (After Mall.) 



External Carotid 



ffulJbus Arteriosus '■ 



PdlmooaryAr-tery ' 

 SabclaviarfArtertis 



Interna I Nummary — \ 

 l>eep£pigastric Arteries ) 



femoral Artery -.^ 



UmbHicol Artery, 



tomosis now occurs resulting in the single basilar and anterior spinal arteries of the adult. The 

 posterior communicating, proximal portions of the posterior cerebrals, the fourth part of the verte- 

 brals, and the right and left roots in the anterior spinals of the adult alone retain the primitive 

 arrangement and testify to the double nature of the original anastomosis. Asymmetry in the 

 vertebrals and other irregularities in the adult can usually be explained on developmental 

 grounds. The postneural anastomosis, which joins the preneural at about the first cervical seg- 

 ment, retains its bilaterality throughout to form the paired posterior spinal arteries of the adult. 

 The lateral segmental arteries take origin from the aorta in series, intermediate in position 

 between the dorsal and ventral segmentals. They reacli their fullest development in embryos 

 of about 8 mm., when they extend from the seventh cervical to the twelfth thoracic segment and 

 supply the mcsonephros. At this stage Broman found twenty arteries on each side, many of 

 which were non-segmental. As the suprarenals and gonads develop, they each receive branches 

 from several mesonephric arteries. The latter arteries now undergo rapid retrogression and 

 tlie suprarenal and gonadic branches are shifted caudally through the mesonephric series to 

 newly formed (non-segmental) arteries opposite tlie ui)per lumbar segments. Finally there 

 remain three suprarenal arteries opposite the twelfth thoracic and first and second lumbar 

 segments and a gonadic artery {ovarian or internal spermatic of tlie adult) opposite the third 

 lumbar segment. All of the.se ve.ssels now appear to be direct branches from the aorta. Of the 

 tliree suprarenal branches, the upper and lower each gives a large branch to the diaphragm and 

 kidney respectively and become the inferior phrenic and re7ial arteries of the adult. The middle 

 becomes the middle suprarenal of the adult. Felix puts a .somewhat different interpretation upon 

 the origin of the vessels persisting in the lumbar region after the disappearance of the thoracic 

 mesonephric arteries, lie finds in an embryo of 18 mm. nine arteries on eitlier side, extending 

 from the nintli thoracic to tlie third lunil)ar segment, all of which he looks upon as mesonephric. 

 These he chissifies into three groups: — Cranial, which roach the mesonephros by passing dorsal 

 to the suprarenal; caudal which j)a.ss ventral to the suprarenal, and middle which pass through 

 it. Inasmuch as the arteries anastomose in the mesonephros there is great liability to variation 

 in the number and position of the sterns which persist in the adult. The suprarenal arteries 



