COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 263 



voluted, and pass laterally to the ovaries. In its passage along the mid-dorsal 

 line the aorta gives off paired lumbar arteries at segmental intervals. These 

 are found by loosening the aorta and looking on its dorsal surface. Posterior 

 to the genital arteries the inferior mesenteric arises as an unpaired visceral branch 

 and passes to the descending colon and rectum, running in the mesocolon. In 

 the mesocolon it forks into the left colic artery passing craniad along the descend- 

 ing colon and the superior hemorrhoidal artery passing caudad to the posterior 

 part of the descending colon and the rectum. 



Add these vessels to the drawing of the aorta. 



The digestive tract may now be removed and discarded, leaving the end of 

 the large intestine in place. Hold the stump of the colon together with the 

 urinary bladder and in female specimens the uterus (the forked coiled tube at 

 the posterior end of the peritoneal cavity) back against the pubes and follow the 

 aorta farther. Near the end of the peritoneal cavity it forks into the two common 

 iliac arteries in the rabbit; in the cat it gives off a pair of external iliac arteries 

 followed shortly by a pair of internal iliac arteries. Anterior to this place in 

 the cat, or in the rabbit at the level of the fork or from the common iliac arteries, 

 a pair of iliolumbar arteries arises and passes laterally along the body wall. 

 The iliolumbar artery divides into an anterior branch, which passes forward 

 toward the kidney, and a posterior branch, which extends to the thigh. 



The two common iliac arteries in the rabbit soon fork into an anterior external 

 iliac and a posterior internal iliac. In the cat the external and internal iliacs 

 arise separately from the aorta, the latter immediately posterior to the former. 

 After giving rise to the iliacs the aorta continues in the mid-dorsal line as the 

 small median sacral or caudal artery, lying halfway between the two internal 

 iliacs. In the cat this vessel arises from the fork of the internal iliacs. In the 

 rabbit it springs anterior to the forking of the aorta, from the dorsal surface of 

 the latter; its origin is concealed by the postcaval vein and will be seen later. 

 The sacral artery supplies the sacral region and the tail. 



Follow the external iliac. It passes laterocaudad out of the peritoneal 

 cavity, in the rabbit to the dorsal side of the inguinal ligament. As it passes 

 through the abdominal wall or shortly beyond the wall it gives rise to the deep 

 femoral artery (cat) or the inferior epigastric (rabbit). In the cat this vessel 

 gives off branches into the thigh, while these are lacking in the rabbit. In both 

 animals the following branches are present: branches into the mass of fat between 

 the thighs and into the external genital organs, of which one branch, in male 

 specimens, constitutes the external spermatic artery; and the main vessel then, 

 as the inferior epigastric artery, turns craniad and ascends in the abdominal 

 wall, running along the inner surface of the rectus abdominis muscle. It anas- 

 tomoses with the superior epigastric artery. In the rabbit there arises, either 

 from the inferior epigastric at the origin of the latter from the external iliac or 

 from the external iliac itself near by, the superficial epigastric artery which extends 



