THE ABDOMEN
Arterial supply of the small intestine (see Figs 5.21 and 5.22) The entire small intestine is supplied by the superior mesenteric artery which arises from the aorta at the L1 vertebral level. Jejunal and ileal branches arise from the left of the main trunk. These branches link with one another in a series of arcades, which are usually single in the jejunum but number up to five in the distal ileum. The arteries that enter the intestinal wall - the vasa recta - are end arteries. Venous drainage of the small intestine Veins from the small intestine drain to the superior mesenteric vein, which in turn drains to the portal vein (see Figs 5.44 and 5.45). Lymphatic drainage of the small intestine Lymph drainage is to the superior mesenteric group of preaortic nodes. Meckel's diverticulum This is a diverticulum that projects from the antimesenteric border of the lower ileum. It represents the persistent intestinal end of the vitellointestinal duct, which connects the yolk sac to the primitive digestive tube in early fetal life and is found in about 2% of subjects. Meckel's diverticulum is said to be 2 inches (5 cm) long and situated 2 feet (60 cm) from the ileocaecal valve ('rule of 2s': 2% incidence, 2 inches, 2 feet, male:female 2:1, symptomatic before 2 years). In fact, its length is very variable and may be merely a bulge on the ileal wall or be up to 15 cm long. Its distance from the caecum may also vary from 15 cm to 3.5 m. The apex of the diverticulum may be adherent to the umbilicus or attached to it by a fibrous cord. It may have gastric, hepatic or pancreatic tissue at its apex. The vitellointestinal duct may also rarely persist as a fistula from the small intestine to the umbilicus; as a cyst along the path of the duct; or as a raspberry tumour of the umbilicus, which is the pouting red mucosa of the persistent extremity of the duct. Radiological features of the small intestine Plain films of the abdomen Gas and fluid levels are often visible in normal loops of small intestine. Up to five fluid levels in loops of 2.5 cm diameter or smaller, or two loops wider than this, may be seen on a normal radiograph. Jejunal loops are distinguished from ileal loops by their position, with the former being in the left upper abdomen
whereas the ileal loops tend to be in the lower abdomen and the right iliac fossa. In radiographs of intestinal obstruction the central position of dilated small-bowel loops helps distinguish them from loops of dilated colon. Other identifying features of small-intestinal loops include the circular valvulae conniventes, as distinct from the incomplete septa formed by colonic haustra (see the section on the colon). Barium studies of the small intestine (see Fig. 5.16) The small intestine may be imaged using a variety of contrast techniques. In a barium follow-through examination the barium is taken orally and imaged as it passes through to the caecum. In a small-bowel enema (or enteroclysis) a tube is passed to the duodenojejunal flexure and barium is passed directly into the small intestine. Normal upper limits of diameter are higher for the distended bowel than for the relaxed state. Thus diameters of up to 4 cm in the jejunum and 3 cm in the ileum are normal in small-bowel enemas. Normal valvulae conniventes may be up to 2 mm thick in the jejunum and 1 mm in the ileum. The valvulae conniventes may be absent in the ileum when it is distended, giving it a featureless appearance. Computed tomography (Figs 5.2-5.4; see also Fig. 5.60b) Oral contrast is used to distinguish normal loops of small intestine from abdominal masses. Loops of small intestine fill most of the middle abdomen and the upper pelvis. When adequately filled with oral contrast the thin wall of normal jejunum is almost imperceptible. Fine, transversely thickened areas due to the valvulae conniventes may be seen. These are seldom seen in the ileum. The mesentery and its vessels and fat may be easily seen. Lymph nodes are frequently visible in the mesentery. Angiography (Fig. 5.21) Selective injection of the superior mesenteric artery demonstrates the jejunal and ileal branches and arterial arcades. The mesenteric vessels can also be readily identified on contrast-enhanced CT and angiographic MR sequences. THE ILEOCAECAL VALVE (Fig. 5.18) The distal ileum opens into the medial and posterior aspect of the large intestine at the junction of the caecum and the ascending colon. Two horizontal crescentic folds of mucosa and circular muscle project into the lumen on the colonic side. These folds are extended laterally as the frenula of the valve. Some thickening of the circular muscle of the ileum at the junction acts as a sphincter.
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