Page 94 - Clinical Anatomy

ECA2 7/18/06 6:42 PM Page 79

The gastrointestinal tract 79

•◊◊splenic flexure;
•◊◊descending colon (9–12in (22–30cm));
•◊◊sigmoid colon (5–30in (12–75cm), average 15in (37cm));
•◊◊rectum (5in (12cm));
•◊◊anal canal (1.5in (4cm)).
The large bowel may vary considerably in length in different subjects;
the average is approximately 5 feet (1.5m).
The colon (but not the appendix, caecum or rectum), bears characteris-
tic fat-filled peritoneal tags called appendices epiploicae scattered over its
surface. These are especially numerous in the sigmoid colon.
The colon and caecum (but not the appendix or rectum) are marked by
the taeniae coli. These are three flattened bands commencing at the base of
the appendix and running the length of the large intestine to end at the rec-
tosigmoid junction. They represent the great bulk of the longitudinal
muscle of the large bowel; because the taeniae are about a foot shorter than
the gut to which they are attached, the colon becomes condensed into its
typical sacculated shape. These sacculations may be seen in a plain radi-
ograph of the abdomen when the large bowel is distended and appear as
incomplete septa projecting into the gas shadow. The radiograph of dis-
tended small intestine, in contrast, characteristically has complete trans-
verse lines across the bowel shadow due to the transverse mucosal folds of
the valvulae conniventes.

Peritoneal attachments

The transverse colon and sigmoid are completely peritonealized (the
former being readily identified by its attachment to the greater omentum).
The ascending and descending colon have no mesocolon but adhere
directly to the posterior abdominal wall (although exceptionally the
ascending colon has a mesocolon). The caecum may or may not be com-
pletely peritonealized, and the appendix, although usually free within its
own mesentery, occasionally lies extraperitoneally behind caecum and
ascending colon or adheres to the posterior wall of these structures.
The rectum is extraperitoneal on its posterior aspect in its upper third,
posteriorly and laterally in its middle third and completely in its lower
third as it sinks below the pelvic peritoneum.

The appendix
The appendix arises from the posteromedial aspect of the caecum about
1in (2.5cm) below the ileocaecal valve; its length ranges from 0.5in (12mm)
to 9in (22cm). In the fetus it is a direct outpouching of the caecum, but
differential overgrowth of the lateral caecal wall results in its medial
displacement.
The position of the appendix is extremely variable—more so than that of
any other organ (Fig. 60). Most frequently (75% of cases) the appendix lies
behind the caecum. The appendix is usually quite free in this position
although occasionally it lies beneath the peritoneal covering of the caecum.
If the appendix is very long, it may actually extend behind the ascending