892 THE VASCULAR SYSTEM. 
primitive condition there was a pair in each segment of the body ; many of these disappear, how- 
ever, and the series is only represented in the adult by the suprarenal, the renal, and the sperm- 
atic or ovarian arteries—possibly, also, by some of the branches of the internal iliac arteries. 
The splanchnic segmental arteries arise in the embryo from the ventral aspects of the 
primitive dorsal aorte, and are distributed to the walls of the alimentary canal. ‘They anasto- 
mose with their fellows in front and behind in the dorsal wall of the gut; those in front of 
the umbilicus also communicate together on the ventral wall of the gut, whilst those behind the 
umbilicus terminate ventrally in the posterior sections of the ventral aortee. 
After the fusion of the dorsal longitudinal trunks to form the descending aorta, the origins of 
the splanchnic arteries in each segment fuse into a common stem, or either the right or left artery 
altogether disappears, whilst at a later period the majority of the splanchnic segmental arteries 
lose their direct connexion with the descending aorta; those which retain their connexion are 
the left bronchial arteries, the esophageal branches of the aorta, the coeliac axis, and the superior 
and inferior mesenteric arteries, the three latter vessels greatly increasing in size. 
The coronary or gastric branch of the cceliac axis, as it passes from its origin to the small 
curvature of the stomach, represents a right splanchnic artery ; the remainder of the coronary 
artery and the pyloric branch of the hepatic are remnants of the ventral anastomoses between the 
splanchnic arteries in front of the umbilicus. 
The splenic artery is a branch given off from a splanchnic artery to an organ developed in the 
gastric mesentery, and the hepatic is a branch from the ventral splanchnic anastomosis 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 the dorsal anastomoses on the gut wall. 
THE AORTA, 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, the common and internal iliacs, and the pulmonary arteries, are all modified portions 
either of the primitive aorte 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. i 
It will be sufficient, therefore, to point out that the primitive aortee are to be regarded — 
as the greatly enlarged pre-central or pre-vertebral longitudinal anastomoses between the — 
successive segmental arteries of each side; obviously, therefore, each primitive aorta, like — 
the rest of the longitudinal anastomoses, consists chiefly of intersegmental elements. The origins 
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of the segmental vessels only enter into its formation in so far as they connect the inter- 
segmental vessels together, and so complete the longitudinal anastomoses. 
The first cephalic aortic arches and the primary caudal arches are simply portions of the 
primitive aorte. The other aortic arches have possibly a different morphological significance, 
but their exact nature is not definitely settled. 
The second, third, fourth, and fifth cephale aortic arches of each side are developed in the un- 
divided mesoderm of the head region behind the first arch. They spring from the anterior part of 
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 resemble segmental vessels. Behind the umbilicus some of the segmental 
splanchnic arteries pass from the dorsal to the ventral aorta in the splanchnic mesoderm on the — 
wall of the alimentary canal. In addition to the vessels already mentioned, there are given off 
from the ventral aortee 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 
thyroid, the thyroidea ima, and the terminal branches of the hypogastric arteries. 
Iliac Arteries and their Branches.—The common iliac arteries are undoubtedly formed 
from the primitive aortic longitudinal vessels; they are simply those portions of the right 
and left primitive aortee respectively which lie immediately beyond or caudal to the per- 
manent descending aorta. The direct continuation of each is the primary caudal arch, 
which forms the origin of the superior vesical artery, and is prolonged as the hypogastric 
artery; these continuous channels are for the most part made up of intersegmental vessels. 
So also are the permanent vessels into which they are transformed. Reference has already been 
made to the fact that the primary caudal arch almost entirely disappears, and that a secondary 
caudal arch is developed in lieu of it (p. 878). 
The internal iliac arteries are almost entirely formed from the secondary caudal arches. The 
primary caudal arch, beyond doubt, is not a segmental vessel; the secondary arch may be, but 
this is still unproved. he branches of the internal iliac artery which represent offsets of the 
“caudal arch” portion of the primitive aorte are arranged in two groups—(1) a visceral set 
which supplies the walls of the hind-gut and the genital organs, and (2) a parietal set which is 
distributed to the body wall and to the hind-limbs. The branches distributed to the gut prob- 
ably represent the segmental splanchnic vessels given off from the dorsal longitudinal vessels ; 
those to the genital organs appear to correspond with the intermediate visceral branches, for they 
are distributed to organs derived from the intermediate cell mass. 
The parietal set are to be regarded as modified somatic segmental branches of the dorsal 
longitudinal trunks. The lateral sacral arteries which belong to this group represent, in the 
ereater parts of their extent at least, the pre-costal anastomoses. 
