INTERSEGMENTAL ARTERIES AND THEIR ANASTOMOSES. 1047 



persistent pre-costal anastomoses, and the ascending cervical artery belongs to the same series of 

 vessels. The vertebral artery, which appears as a branch of the subclavian in the adult, is, 

 morphologically, somewhat complex. The first part represents the dorsal branch of the seventh 

 somatic intersegmental artery ; the second part, that passing through the cervical transverse 

 processes, consists of the persistent post-costal anastomoses between the dorsal branches of the 

 first seven intersegmental arteries ; a third part, that lying on the arch of the atlas, is the spinal 

 branch of the first somatic intersegmental artery and its neural continuation ; whilst finally the 

 upper part of the vertebral artery, the part in the cranial cavity, appears to represent a pro- 

 longation of the pre-neural anastomoses, which still farther upwards are probably represented by 

 the basilar artery. As already stated, the post-costal anastomoses below the seventh intersegmental 

 artery occasionally persist, and in such cases the vertebral may lose its connexion with the sub- 

 clavian, and spring from one or other of the posterior branches of the upper intercostal arteries. 



The profunda cervicis artery is to be regarded as a remnant of the post-transverse longi- 

 tudinal anastomoses. 



The origin of the seventh somatic intersegmental artery from the dorsal longitudinal trunk 

 is, at first, some distance caudal to the sixth aortic arch, but, simultaneously with the elongation of 

 the neck and the retraction of the heart into the thoracic region, it is shifted cranialwards until 

 it is opposite the dorsal end of the fourth aortic arch. 



The middle sacral artery is formed by the fusion of two vessels, each of which springs from the 

 dorsal surface of the aorta. It is regarded as the direct continuation of the descending aorta. 



The lateral or intermediate visceral arteries supply the organs derived from the inter- 

 mediate cell mass. They form a somewhat irregular series of vessels in the adult, but pre- 

 sumably in the primitive condition there was a pair in each segment of the body ; many of 

 these disappear, however, and the series is only represented in the adult by the suprarenal, 

 the right renal, part of the left renal, and the testicular or ovarian arteries possibly, also, by 

 some of the branches of the hypogastric arteries. 



The splanchnic arteries arise in the embryo from the ventral aspects of the primitive 

 dorsal aortse, and are not strictly either segmental or intersegmental in arrangement. They are 

 distributed to the walls of the alimentary canal. Each anastomoses with its immediate neigh- 

 bours on the dorsal wall and the ventral walls of the gut. 



After the fusion of the dorsal longitudinal trunks to form the descending aorta, the roots of 



each pair of the splanchnic arteries fuse into a common stem, or either the right or left artery 



altogether disappears, whilst at a later period the majority of the splanchnic arteries lose their 



, direct connexion with the descending aorta ; those which retain their connexion are the cceliac 



: artery and the superior and inferior mesenteric arteries. 



The bronchial and cesophageal arteries are later formations. They appear to correspond morpho- 

 . logically with the more primitive splanchnic arteries, but the developmental history is not known. 



The left gastric branch of the coaliac artery, as it passes from its origin to the small 

 curvature of the stomach, represents a right splanchnic artery ; the remainder of the left gastric 

 artery and the right gastric branch of the hepatic are remnants of the ventral anastomoses between 

 r the splanchnic arteries cephalwards of the umbilicus. 



The splenic artery is a branch given off from a splanchnic artery to an organ developed in the 

 mesogastrium, and the hepatic is a branch from the ventral splanchnic anastomoses to the 

 hepatic diverticulum from the wall of the duodenal portion of the fore-gut. 



The superior and inferior mesenteric arteries represent at their origins splanchnic branches, 

 and in the remainder of their extent they represent the dorsal anastomoses on the gut wall. 



THE A'ORTA, PULMONARY ARTERY, AND OTHER CHIEF STEM VESSELS. 



The heart and the majority of the great arterial trunks of the body, including the aorta, the 

 innominate, part of the right subclavian, the common, external, and greater parts of the internal 

 carotids, and the pulmonary arteries, are all modified portions either of the primitive aortae or 

 of the aortic arches. The developmental changes, which result in the formation of the vessels 

 named, are described in the preceding chapter, and the morphology of these vessels is obviously 

 the same as that of the trunks from which they are derived. 



It will be sufficient, therefore, to point out that the primitive aortas may be regarded 

 as the greatly enlarged pre-central or pre-vertebral longitudinal anastomoses between the 

 successive intersegmental arteries of each side ; obviously, therefore, each primitive aorta, like 

 the rest of the longitudinal anastomoses, consists chiefly of segmental elements. The origins of 

 the intersegmental vessels enter into its formation only in so far as they connect the segmental 

 vessels together, and so complete the longitudinal anastomoses. 



The first cephalic aortic arches are simply portions of the primitive aortee. The other 

 aortic arches have possibly a different morphological significance, but their exact nature is not 

 definitely settled. 



The second, third, fourth, fifth, and sixth cephalic aortic arches of each side are developed in 

 the undivided mesoderm of the head region, caudal to the first arch. They spring from the part 

 ol the primitive aorta which, after the head fold is formed, lies on the ventral aspect of the fore- 

 gut, and they extend, at the side of the pharyngeal part of the fore -gut, to the dorsal aorta. Thus, 

 in some respects they may be looked upon as segmental vessels. In addition to the vessels already 

 mentioned, there are given off from the ventral aortse and the aortic arches a series of branches 

 which supply ventral and lateral diverticula from the alimentary canal ; these are represented 

 ! in the adult by the superior thyreoid, the thyreoidea ima. 



