MORPHOLOGY OF THE ARTERIAL SYSTEM. 



505 



tongue developed in the concavity of the mandibular arch; and the facial and internal 

 maxillary are respectively superficial and deep arteries extending into the foetal mandibular 

 and maxillary processes. The posterior auricular artery, lying over the styloid process, is in 

 the position of the second arch, and the occipital is probably an offset from the same ; the 

 origin of the occipital artery from the internal carotid, which is sometimes observed, may be 

 explained as resulting from the persistence of the dorsal connection of the second arch. 



The primitive dorsal aortas become fused for the greater part of their extent into the single 

 descending aorta, from which there are given off 1, a series of metameric somatic or parietal 

 arteries on each side to the body- wall, 2, a smaller number of azygos splanchnic or visceral 

 branches which pass forwards in the primitive mesentery to the alimentary canal, 3, a set of 

 paired arteries to the suprarenal bodies, and the renal and genital glands, and^4 3 mediastinal 

 and phrenic branches to the septa of the body-cavity. 



1. Somatic arteries. The typical arrangement of these vessels is seen in the upper aortic 

 intercostal arteries, and less perfectly in the lower intercostals and the lumbar arteries. Each 

 enters its body-segment with the corresponding spinal nerve, and like that divides on the side 

 of the vertsbra into a dorsal and a ventral branch. The dorsal branch passes backwards 



Fig. 397. SCHEME OP THE TYPICAL 



ARRANGEMENT AND COMMUNICA- 

 TIONS OF THE BRANCHES OF THE 

 AORTA. (Gr. D. T.) 



Longitudinal anastomoses : 1, pre- 

 costal ; 2, postcostal ; 3, postverte- 

 bral ; 4, anterior neural ; 5, posterior 

 neural ; 6, ventral somatic ; 7, dorsal 

 splanchnic. 



between the transverse processes 

 of the vertebras, furnishes a spinal 

 offset to the interior of the spinal 

 canal, and is distributed to the 

 dorsal muscles and integument. 

 The ventral branch is continued 

 in the intercostal space to the 

 front of the body, and gives off in 

 its course a lateral perforating off- 

 set with the lateral branch of the 

 nerve : from such lateral branches 

 the main arteries of the limbs are 

 derived. At the fore part of the 

 intercostal space an anterior per- 

 forating branch accompanies the 

 termination of the nerve. The 

 adjacent segmental arteries are 



united by longitudinal anastomoses, which, although for the most part small, and not unfre- 

 quently wanting, in certain regions attain a greater development and give rise to trunks 

 through which the blood-supply is furnished to the several structures of the part, the 

 primitive origins being more or less completely obliterated. The chief of these are 1, the 

 precostal anastomosis in front of the neck of the rib, 2, the postcostal anastomosis between 

 the neck of the rib and the transverse process, 3, the post vertebral anastomosis between the 

 dorsal branches behind the transverse process, 4 and 5, the single anterior neural and the 

 paired posterior neural anastomoses formed by the spinal branches on the spinal cord, and 6, 

 the central somatic anastomosis uniting the anterior ends of the ventral branches, and giving 

 rise to the internal mammary and epigastric arteries. 



In the neck, as a consequence of the withdrawal of the aortic trunk from this region with 

 the descent of the heart and lower arches, the upper six segmental arteries, which were present 

 in the early embryo, have disappeared, and the aortic connections of the last cervical and the 

 upper two thoracic arteries are also obliterated. The seventh artery however persists, 

 and its ventral division, becoming greatly developed, gives rise to the first part of the sub- 

 clavian artery (with the exception of the small proximal portion of the artery of the right 

 side formed from the fourth arch), the continuation of which, passing between the scaleni. is 

 the lateral perforating branch, while the prolongation of the segmental vessel is represented 

 by the portion of the internal mammary descending to the first rib-cartilage, where it passes 

 into the ventral longitudinal anastomosis. The ascending cervical artery is formed from the 

 precostal anastomoses of the suppressed arteries ; and the vertebral trunk, whioh at its origin 



