666 BLOOD-VASCULAR 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 basé of the czecum. 
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, 7. e., about five or six inches (ca. 12-15 em.) 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 hemorrhoidal artery 
(A. heemorrhoidalis 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. spermatic interne), right and left, 
are long slender arteries which arise from the aorta near the posterior mesenteri¢ 
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 spermatie 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. 
2 Also termed the left or retrograde colic artery. 
8 Variations 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. 
