CIRCULATORY ORGANS. 



285 



merit 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 cceliac 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 to 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 



FIG. 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 7 ; 

 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; il, iliac artery and vein; 

 j, jugular; Iv, liver; m, mv, mesenteric artery and vein; pa, pulmonary artery; pcd, post- 

 cardinal; pcv, postcava; pv, hepatic portal vein; r, 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 reproductiye 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 



