The Mesentery

Abstract

CONSIDERED anatomically, the mesentery represents two layers of peritoneal lining separated by loose connective tissue in which branches of arteries, veins, and lymphatics extend to the intestinal tract. Therefore, broadly speaking, we should include the following structures: liggamenta, gastro-hepaticum, hepatoduodenale, gastro-lienale, gastro-colicum, bursa omentalis, mesocolon transversum, and sigmoideum, besides the radix mesenterii, which is the chief object of our discussion. In a restricted sense, the mesentery is, then, that part of the duplicated peritoneal lining which extends from the radix mesenterii to the small intestine. It originates on the left border of the second lumbar vertebra and extends obliquely toward the right iliac fossa, ending just above the right sacro-iliac region. All of these structures show numerous anatomic variations, some of which are of clinical importance. To understand this, it is advisable to review briefly their embryologic development. Maldevelopments and Variations.—In the fifth week of embryonic life, the digestive tract is represented by a long duct: the fore-gut is fixed in the mid-line of the body by the ventral and dorsal mesenteries (Hubeny). The mid- and hind-guts are suspended from the posterior body wall only by their dorsal mesenteries: the ventral mesenteries have disappeared. The mesentery of the mid-gut is long, and the intestinal tube forms a long U-loop which is supplied by the superior mesenteric artery. The U-loop cannot move freely in a lateral direction, because its apex is fixed to the umbilicus by means of the vitelline duct. However, it is able to rotate around its base, where the two limbs of the U are close together. After the fifth week the mid-gut increases in length very rapidly: its mesentery becomes fan-shaped and capacious, except at the base of the loop, where it is reduced to a narrow isthmus through which the superior mesenteric artery passes. At the beginning of the third month the U-shaped loop twists around the isthmus from right to left, thus elevating the distal part of the hind-gut, which becomes attached on the left side, as the splenic flexure of the colon. The rest of the colon proximal to this swings over the front of the abdominal cavity from left to right, becoming attached to the structures of the posterior body wall along a line extending from the left to the right kidney, and thence down to the right iliac fossa. This explains why a part of the small gut (second and third parts of the duodenum) is situated behind the transverse colon, while the rest (jejunum and ileum) lies freely in the abdomen, below the transverse colon. It is attached with its radix mesenterii posteriorly along the line described above, extending obliquely from the second lumbar vertebra on the left side to the right sacro-iliac region. The free mesentery is covered on both sides by the peritoneal lining; the attached one, on its anterior surface only.