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The small intestine (Fig. 16.1)
part of the duodenum to enter the root of the mesentery and pass
The small intestine is approximately 6 m long and comprises the duo-
towards the right iliac region on the posterior abdominal wall. Jejunal
denum, jejunum and ileum. A large internal surface area throughout the
and ileal branches arise which divide and re-anastomose within the
small intestine facilitates absorption of digested products. The small • The superior mesenteric vessels (see Fig. 12.3) pass over the third
intestine is suspended from the posterior abdominal wall by its mesen- mesentery to produce arcades. End-artery vessels arise from the
tery which contains the superior mesenteric vessels, lymphatics and auto- arcades to supply the gut wall. The arterial supply to the jejunum con-
nomic nerves. The origin of the mesentery measures approximately 15 sists of few arcades and little terminal branching whereas the vessels to
cm and passes from the duodenojejunal flexure to the right sacro-iliac the ileum form numerous arcades and much terminal branching of end-
joint. The distal border is obviously the same length as the intestine. arteries passing to the gut wall.
No sharp distinction occurs between the jejunum and ileum; however,
certain characteristics help distinguish between them: Small bowel obstruction (Fig. 16.2)
• Excluding the duodenum, the proximal two-fifths of the small intes- Small bowel obstruction (SBO) can occur due to luminal, mural or
tine comprises jejunum whereas the remaining distal three-fifths com- extraluminal factors that result in luminal blockage. Post surgical
prises ileum. Loops of jejunum tend to occupy the umbilical region adhesions and herniae are the most frequent causes. Many cases
whereas the ileum occupies the lower abdomen and pelvis. resolve with conservative measures only; however, if any deterioration
• The mucosa of the small intestine is thrown into circular foldsathe in the clinical picture occurs to suggest intestinal infarction or perfora-
valvulae conniventes. These are more prominent in the jejunum than in tion an exploratory laparotomy is mandatory. The classical X-ray fea-
the ileum. tures of SBO are those of dilated small bowel loops. These can be
• The diameter of the jejunum tends to be greater than that of the ileum. distinguished from large bowel as the valvulae conniventes (present
• The mesentery to the jejunum tends to be thicker than that for the only in the small bowel) can be identified traversing the entire lumen
ileum. whereas the small bowel haustra only partially traverse the lumen.

The upper gastrointestinal tract II 41