Topography

Topography of the fore-stomachs and abomasum is incorporated in descriptions of approach (e.g. left flank laparotomy, traumatic reticulitis). The complex topography of the intestinal tract is considered in this introduction.

The intestinal tract

The small intestine includes the duodenum (cranial, descending, and ascending parts), jejunum, and ileum. The large intestine is comprised of the caecum, ascending colon (proximal loop, spiral loop [centripetal and centrifugal coils], distal loop), transverse colon, descending colon, and rectum.

Position and course

The muscular pylorus lies fairly fixed in position supported by the origin of the lesser omentum dorsally and the greater omentum ventrally, and situated level with the costochondral junction of ribs 9 and 10 on the right side. The cranial duodenal loop passes craniodorsal, initially freely mobile, but with the next portion firmly adherent to the visceral hepatic surface. It curves in an S-shaped manner near the bile and pancreatic duct openings and becomes the descending loop, which is suspended dorsally by the meso-duodenum, and the loop passes caudally in the dorsal and right lateral part of the abdominal cavity. Superficial and deep parts of the greater omentum attach to the ventral surface of the descending loop of the duodenum.

The duodenum turns cranial at the caudal flexure, where it is attached to the descending colon by the duodeno-colic ligament, and becomes the ascending loop to pass cranial to the left side the mesenteric root. It turns to the right side of the root to become the jejunum. The jejunum, 35–50 m long, comprises a mass of tight coils at the edge of the mesentery. The greatest intestinal mass is formed by these heaped coils of jejunum. The mesentery of the proximal and middle sections is short, that of the distal part and that attached to the ileum are longer, forming a mobile section that lies caudal to the supra-omental recess.

The ileum comprises a convoluted proximal segment and a distal straight part. The junction of jejunum and ileum is the point where the cranial mesenteric artery ends, and the cranial limit of the ileocaecal fold. The ileum is attached to the caecum ventrally, the orifice lying obliquely on the ventral surface of the caecum, and readily identified in adulthood due to the fat pad overlying it.

The caecum is a mobile sac, with the blind-end directed caudally. Cranially the caecum is continuous with the proximal loop of the ascending colon. The short caecocolic fold attaches the caecum to the colon dorsally. The caecum often extends caudally to the limits of the supraomental bursa. The proximal loop of ascending colon passes cranial to the level of T12 then turns caudally to pass dorsally to the first segment. It again turns craniad, but now to the left of the mesentery, and then ventrally to become the spiral loop of the ascending colon. The arrangement comprises two centripetal, followed by two centrifugal coils.

The central flexure is the mid-point and the change in direction of the spiral colon. The distal portion of the spiral colon is normally adjacent to the ileum. The distal loop of the ascending colon passes caudad along the left side of the mesentery, around which it turns to run craniad again, adjacent to the proximal colon. It then becomes the transverse colon, which passes from the right to the left side, around the cranial edge of the cranial mesenteric artery. The descending colon proceeds caudad along the dorsal surface of the abdomen, attached by the mesocolon. The mesocolon is rather elongated at the level of the duodenocolic ligament, affording it some mobility. The descending colon terminates in the rectum, which lies entirely intrapelvic.

The relative shortness of the mesentery means that exteriorization of the intestine is difficult in many areas. Vessels and lymph nodes within the mesentery are hard to identify due to fat deposition. The ascending duodenum, proximal and distal loops of ascending colon, and the cranial portion of the descending colon lie close to one another due to the near-fusion of their mesenteries. The cranial mesenteric vessels supply the small and large intestine, except for parts of the duodenum and colon.

The greater omentum passes from its origin on the duodenum, pylorus, and greater curvature of the abomasum, encircles the intestinal mass, and inserts on the left longitudinal groove of the rumen (superficial part), while the deep
part passes similarly ventral and to the left, to attach to the right longitudinal groove of the rumen. These two parts are fused caudally forming the caudal fold. The lesser omentum extends from the esophagus along the reticular groove and omasal base to attach to the lesser curvature of the abomasum, and covers most of the parietal surface of the omasum.