570 BLOOD-VASCULAR SYSTEM OF THE HORSE 



(d) The left gastro-epiploic artery (A. gastroepiploica sinistra) is the continua- 

 tion of the splenic artery. It passes to the right in the great omentum, parallel 

 with the greater curvature of the stomach, and anastomoses with the right gastro- 

 epiploic. It gives off branches to the greater curvature of the stomach, and twigs 

 to the omentum. 



II. The anterior or great mesenteric artery (A. mesenterica cranialis) arises 

 from the ventral face of the aorta at the first lumbar vertebra. It is a large un- 

 paired trunk, about an inch (ca. 2 to 3 cm.) in lengih, which passes ventrally between 

 the vena cava and the left adrenal into the root of the great mesentery, when it 

 divides into three branches — left, right, and anterior.^ 



1. The left branch gives off at once about fifteen to twenty arteries of the 

 small intestine (Aa. intestinales).^ These come off close together and pass in 

 divergent fashion between the layers of the great mesentery, each dividing into 

 two branches which anastomose with adjacent branches to form a series of arches. 

 In the anterior part of the series secondary arches are formed by the union of 

 branches given off from the primary set of arches. From the convex side of these 

 arches terminal branches pass to the wall of the small intestine, in which they 

 ramify and form a vascular network. They are accompanied by satellite veins and 

 by nerves and lymph-vessels. The first artery anastomoses with the pancreatico- 

 duodenal, and the last with the ileal branch of the great mesenteric artery. Branches 

 are supplied to the mesenteric lymph glands. 



2. The right branch (A. ileo-cseco-colica) might be regarded as the continuation 

 of the trunk. It runs downward and a little forward and to the right and gives 

 off the ileal, the two csecal, and the ventral colic arteries. 



(1) The ileal or ileo-caecal artery (A. ilea) passes in retrograde fashion along 

 the terminal part of the ileum and unites with the last branch of the left division. 



(2) The external or inferior caecal artery (Ramus csecalis lateralis) passes be- 

 tween the caecum and the origin of the colon and runs on the external muscular 

 band of the caecum to the apex, where it anastomoses with the internal artery. 

 Besides numerous collaterals to the caecum, it gives off the artery of the arch, 

 which passes along the lesser curvature of the base of the caecum and runs on the 

 outer face of the origin of the great colon. 



(3) The internal or superior caecal artery (Ramus caecalis medialis) passes 

 along the inner band to the apex of the caecum, where it anastomoses with the 

 external csecal artery. 



(4) The ventral, right, or direct coHc artery (A. colica ventralis) runs along the 

 opposed surfaces of the ventral (first and second) 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 caecum. 



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

 middle colic arteries. 



(1) The dorsal, left, or retrograde colic artery (A. colica dorsalis) is a large vessel 

 which passes along the dorsal (fourth and third) 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 anasto- 

 motic branch to the dorsal colic artery. 



1 In the great majority of sulijects this vessel and some of its branches are the seat of more 

 or less extensive verminous aneurysm, produced by the Sclerostomum armatum. In the author's 

 experience an entirely normal specimen is quite exceptional in adult horses. 



- Tlie left branch is a descriptive convention rather than a reality, since the arteries of the 

 small intestines spring from the mesenteric trunk either directly or by short common stems 

 wth an adjacent vessel. 





