CIRCULATORY ORGANS. 285 
ment with the omphalomesenteric arteries (p. 276) and goes to the ante- 
rior part of the intestine; while frequently an inferior mesenteric artery 
is distributed to the posterior part of the digestive tract. The superior 
mesenteric may fuse with the coeliac to form the cceliac axis while not 
infrequently other mesenteric arteries may be developed. 
The hypogastric arteries, already mentioned, need further notice. 
These primitively connect the dorsal aorta with the subintestinal vein 
in the neighborhood of the anus, and later give off vessels tq the region 
of the rectum. When, as in all classes, from the amphibia upward, a 
urinary bladder is developed from the rectal (cloacal) region, the 
Fic. 289.—Diagram of vertebrate circulation based on a urodele. Arteries cross- 
lined; veins black except the pulmonary vein, white. av, abdominal vein; a, cceliac artery; 
ca, cv, caudal artery and vein; d, dorsal aorta; ec, external carotid; g, gonad; h, hepatic 
vein; ha, hepatic artery; hy, hypogastric artery; ic, internal carotid; i, iliac artery and vein; 
j, jugular; /v, liver; m, mv, mesenteric artery and vein; pa, pulmonary artery; ped, post- 
cardinal; pcv, postcava; pu, hepatic portal vein; 7, rectal artery; ra, renal advehent vein; 
sc, subclavian artery and vein. 
hypogastrics form its blood supply, these vessels being the vesical 
arteries. In the amniotes the distal end of the anlage of the bladder 
forms a foetal structure known as the allantois, described in another 
section (p. 318), and parts of the vesical arteries are carried out as 
allantoic arteries (fig. 273), into the new formation. Since these 
pass through the umbilicus, they are also known as the umbilical 
arteries. Later, when the umbilicus disappears, the allantoic arteries 
are lost and only the rectal and vesical arteries remain of the hypo- 
gastric trunks. 
The arteries going to the excretory and reproductive organs are 
paired and, in the more primitive vertebrates show a marked metamer- 
ism. They are best described in details along with the urogenital 
structures in a subsequent section. It may be mentioned here that the 
metamerism is well shown in the nephridial or renal arteries going to 
