666 BLOOD-VASCULAE SYSTEM OF THE HORSE 



artery (A. colica ventralis)^ runs along the dorso-medial bands of the opposed 

 surfaces of the ventral parts of the great colon to the pelvic flexure, where it unites 

 with the dorsal colic artery. It supplies the ventral parts of the great colon and 

 sends a branch to the base of the ceecum. 



3. The anterior branch divides after a very short course into the dorsal and 

 middle colic arteries. (1) The dorsal colic artery (A. colica dorsalis)- is a large 

 vessel which passes along the dorsal parts of the great colon to the pelvic flexure, 

 where it joins the ventral colic artery. (2) The middle colic artery or first artery 

 of the small colon (A. colica media) is a much smaller vessel which passes to the 

 origin of the small colon, enters the colic mesentery, and forms an arch by joining 

 the first branch of the posterior mesenteric artery close to the lesser curvature of 

 the bowel. It sends an anastomotic branch to the dorsal colic artery. 



III. The renal arteries (Aa. renales), right and left, are relatively large vessels 

 which arise from the aorta near the anterior mesenteric. The right artery is the 

 longer of the two. It crosses over the dorsal surface of the vena cava to the right 

 and somewhat forward. At the hilus it divides into several (five to eight) branches; 

 some of these enter the gland at the hilus, while others pass to the ventral surface 

 and enter there. The left artery is short and usually arises a little further back; 

 it passes directly outward to the kidney and is then disposed like the right one. 

 Small collateral branches are supplied to the ureters, the perirenal fat, the renal 

 lymph glands, and the adrenals. The latter also receive small adrenal arteries 

 directly from the aorta. The distribution within the kidney has been described. 



Variations in the renal arteries are frequent. Two or more arteries may occur on one side or 

 both. Accessory arteries are more common on the left side and usually enter the posterior part 

 of the gland. They may arise from the aorta, the external iliac, or the circumflex iliac artery. 



IV. The posterior or small mesenteric artery (A. mesenterica caudalis) is an 

 unpaired vessel which arises from the ventral face of the aorta at the fourth lumbar 

 vertebra, i. e., about five or six inches (ca. 12-15 cm.) behind the origin of the 

 anterior mesenteric artery. It is much smaller than the latter and supplies the 

 greater part of the small colon and rectum. It descends in the colic mesentery 

 and, after a short course, divides into two branches. The anterior branch (A. colica 

 sinistra) gives off three or four arteries which divide and form anastomotic arches 

 close to the bowel. The first arch is formed by union with the middle colic branch of 

 the anterior mesenteric. The posterior branch or anterior haemorrhoidal artery 

 (A. haemorrhoidalis cranialis) passes backward in the upper part of the mesentery 

 and the mesorectum and terminates near the anus by anastomosing with the 

 internal pudic artery. Three or four of its anterior collateral branches form 

 arches. 



V. The internal spermatic arteries (Aa. spermaticee interna), right and left, 

 are long slender arteries which arise from the aorta near the posterior mesenteric 

 and supply the testicle and epididymis.' Each passes backward in a narrow fold 

 of peritoneum (Plica vasculosa) to the internal inguinal ring and descends through 

 the inguinal canal to the scrotum. In its course in the anterior border of the 

 spermatic cord it forms numerous coils, surrounded by the pampiniform plexus 

 of veins, and associated closely with the spermatic nerves and lymphatics and 

 unstriped muscle-fibers. It passes between the epididymis and testicle, runs in 

 flexuous fashion along the attached border of the latter, turns around the posterior 

 pole, and runs forward on the free border to the anterior pole. The largest branches 

 arise from its ventral part, pass tortuously up either side of the gland, embedded 

 in the tunica albuginea, and give off fine branches to the gland substance. Small 



1 Also termed the right or direct colic artery. 

 ^ Also termed the left or retrograde colic artery. 



' ^'ariations in the origin of the internal spermatic artery are common. It may arise from 

 the posterior mesenteric, or from the renal, or the two may arise by a short common trunk. 



