stomach and the first part of the duodenum, and proceeds upwards as a free fold towards the liver. Above it is attached along the transverse fissure of the liver, the posterior part of the longitudinal fissure of the liver (ie., that part which contains the obliterated ductus venosus) (Fig. 122), and also to the diaphragm in the immediate vicinity of the œsophagus. It presents a right free margin, which extends from the transverse fissure of the liver to the duodenum, and forms the anterior boundary of the foramen of Winslow. Over the greater part of its extent it is very thin; the two layers are fused, and often it is fenestrated. The two layers, however, become evident along its lines of attachment, and also in the portion adjoining its right free edge. The gastro-splenic omentum, composed of two peritoneal layers continuous with the two anterior layers of the great omentum, forms a short fold, which connects the fundus of the stomach with the spleen. Between its two layers the vasa brevia of the splenic artery gain access to the stomach. The Mesenteries of the Great Intestine.—Under ordinary circumstances these are three in number-viz., the transverse meso-colon, the sigmoid meso-colon, and the meso rectum. The transverse meso-colon (mesocolon transversum) stretches from the transverse colon to the posterior wall of the abdomen, to which it is attached along the antero-inferior border of the pancreas (Pl. I., also Fig. 116, p. 491). It is an extensive fold, formed by the two posterior layers of the great omentum after they have enclosed the colon, and containing between them the blood-vessels which go to this portion of the gut. It is longest in the mesial plane of the body, and shortens as it is traced to the right and to the left, until finally at the two flexures which mark the limits of the transverse colon it ceases to exist as a free fold. The sigmoid meso-colon (mesocolon sigmoideum) is a long mesentery, composed of two layers of peritoneum, which binds the sigmoid flexure of the colon to the left iliac fossa. At its root it is narrow, but it widens out as it is traced to the gut, and it contains within it the blood vessels which supply this portion of the intestine. The line of attachment of its root is oblique, and extends from the highest point of the iliac crest across the iliac fossa to the left sacroiliac articulation where it crosses the iliac vessels (Pl. I., also Fig. 103, p. 444). Towards both ends the fold rapidly shortens, and inferiorly it becomes continuous with the meso-rectum. The loop of intestine which is enclosed in sigmoid meso-colon when held up presents a somewhat pedunculated appearance, and cases of intestinal obstruction through the twisting of this loop of gut around its own base are not unknown. The meso-rectum will be examined in connection with the pelvis. The Mesentery Proper.-By this we mean the extensive fold of peritoneum by which the jejunum and ileum are suspended from the posterior wall of the abdomen. To obtain a proper view of the mesentery proper, it is necessary to throw up the great omentum and the transverse colon over the lower margin of the chest. It is attached along an oblique line, which extends from the left side of the body of the second lumbar vertebra downwards, and to the right into the right iliac fossa (Fig. 103, p. 444). This attachment is called the "root" of the mesentery, and as it is traced downwards it will be seen to cross obliquely the third part of the duodenum, the aorta on the front of the spine, and the psoas muscle (Pl. I.). The "root" of the mesentery is thus, comparatively speaking, narrow (about six inches), but, as the fold approaches the coils of the small intestine, it widens out enormously, so that when it reaches the gut its width equals the length of the jejunum and ileum. This great width is not at first apparent, because the mesentery is thrown into folds like a goffered frill. The coiled condition of the gut is due to this arrangement. The two layers of the mesentery are not in apposition with each other. They are separated by a variable amount of fat, and also certain important structures which lie between them. These are (1) the superior mesenteric vessels and their branches to the jejunum and ileum; (2) the superior mesenteric nerves; (3) great numbers of lymphatic glands and lacteal vessels; (4) the gut itself. Occasional Peritoneal Fossæ.-It is necessary to take notice at this stage of three little peritoneal pockets or blind recesses which are occasionally present in different positions on the posterior wall of the abdomen. The importance of these fossæ depends upon the fact that one or other of them may become the site of a form of hernia, termed retro-peritoneal hernia, through a knuckle of the small intestine becoming engaged within it. From their position they are termed (1) the fossa duodeno-jejunalis: (2) the fossa subcæcalis, and (3) the fossa intersigmoidea. In every abdomen that is opened in the dissecting-room these fosse should be looked for before the peritoneal folds are dissected. The fossa duodeno-jejunalis, if present, will be brought into view by throwing the omentum with the transverse colon upwards over the ribs and drawing the coils of the small intestine well to the right. The little fossa lies on the left side of the commencement of the jejunum. It is bounded below by a free projecting semilunar peritoneal fold, and it passes for a short distance behind the parietal peritoneum. Cases are known in which this small peritoneal pocket has become so distended through the entrance of the small intestine that it has formed a hernial sac into which the whole of the jejunum and ileum has passed. When the fossa subcæcalis exists it will be brought into view by drawing the cæcum forwards and to the right. It is a small peritoneal pocket which ascends for a short distance behind the ileo- cæcal junction. It is right to mention that there are several varieties of this fossa. The fossa intersigmoidea is very rarely seen in the adult. When present it will be found by raising the sigmoid loop of the great intestine. Its mouth lies about the middle of the under surface of the root of the sigmoid mesentery, over the interval between the psoas and iliacus muscles (Henle). Dissection. The structures which are included between the two layers of the mesentery proper and between the two layers of the transverse meso-colon must now be dissected. The great omentum being thrown well up over the lower margin of the chest, remove the entire anterior layer of the mesentery from its root down to where it is attached to the gut. Begin at the upper end of the jejunum at the left side of the second lumbar vertebra, and gradually travel downwards to the lower end of the ileum, stripping off the peritoneum and cleaning the structures exposed. To display all the branches of the superior mesenteric artery it is necessary to remove also the inferior layer of the transverse meso-colon and the peritoneum which proceeds on the posterior wall of the abdomen towards the cæcum and ascending colon. Follow the main trunk of the superior mesenteric artery upwards to its origin from the aorta, by raising the lower border of the pancreas. Superior Mesenteric Artery (arteria mesenterica superior). -The superior mesenteric artery springs from the front of the abdominal aorta about a quarter of an inch below the coeliac axis. At its origin it is covered by the pancreas, and crossed by the splenic vein. Emerging from under cover of the pancreas, it crosses the third part of the duodenum, close to the duodeno-jejunal flexure, and then enters the mesentery proper. Between the two layers of the mesentery the artery is placed at a short distance from the mesenteric root, and pursues a slightly curved course towards the right iliac fossa, where it ends by anastomosing with one of its own branches. The convexity of the curve which it describes is directed to the left, and the concavity to the right. It is accompanied by the superior mesenteric vein, which lies upon its right side, and by the superior mesenteric plexus of nerves which surrounds it closely. The following branches proceed from the superior mesenteric artery : 1. Inferior pancreatico-duodenal. 2. Branches to the jejunum Rami intestini tenuis. and ileum. The inferior pancreatico-duodenal (arteria pancreaticoduodenalis inferior) takes origin from the superior mesenteric under cover of the pancreas. It has a curved course round the head of this gland, between it and the duodenum, and gives branches to both. It ends by anastomosing with the superior pancreatico-duodenal artery. The rami intestini tenuis (arteriæ intestinales) spring from the convexity or left side of the superior mesenteric, and proceed obliquely downwards and to the left, between. |