The lower gastrointestinal tract

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The caecum and colon (Figs 17.1, 12.3, 12.5)
In adults, the large bowel measures approximately 1.5 m. The caecum,
ascending, transverse, descending and sigmoid colon have similar
characteristic features. These are that they possess:
• Appendices epiploicae (Fig. 17.1): these are fat-laden peritoneal tags
present over the surface of the caecum and colon.
• Teniae coli (Fig. 17.1): these are three flattened bands representing
the condensed longitudinal muscular coat of the large intestine. They
course from the base of the appendix (and form a useful way of locating
this structure at operation) to the recto-sigmoid junction.
• Sacculations: because the teniae are shorter than the bowel itself the
colon takes on a sacculated appearance. These sacculations are visible
not only at operation but also radiographically. On a plain abdominal Xray,
the colon, which appears radiotranslucent because of the gas within,
has shelf-like processes (haustra) which partially project into the lumen.
The transverse and sigmoid colon are each attached to the posterior
abdominal wall by their respective mesocolons and are covered entirely
by peritoneum. Conversely, the ascending and descending colon
normally possess no mesocolon. They are adherent to the posterior
abdominal wall and covered only anteriorly by peritoneum.
The appendix (Fig. 17.1)
The appendix varies enormously in length but in adults it is approximately
5–15 cm long. The base of the appendix arises from the posteromedial
aspect of the caecum; however, the lie of the appendix itself is
highly variable. In most cases the appendix lies in the retrocaecal position
but other positions frequently occur. The appendix has the following
characteristic features:
• It has a small mesentery which descends behind the terminal ileum.
The only blood supply to the appendix, the appendicular artery (a
branch of the ileocolic), courses within its mesentery (see Fig. 12.4). In
cases of appendicitis the appendicular artery ultimately thromboses.
When this occurs, gangrene and perforation of the appendix inevitably
supervene.
• The appendix has a lumen which is relatively wide in infants and gradually
narrows throughout life, often becoming obliterated in the elderly.
• The teniae coli of the caecum lead to the base of the appendix.
• The bloodless fold of Treves (ileocaecal fold) is the name given to a
small peritoneal reflection passing from the anterior terminal ileum to
the appendix. Despite its name it is not an avascular structure!
Appendicectomy is performed most commonly through a grid-iron
muscle-splitting incision. The appendix is first located and then delivered
into the wound. The mesentery of the appendix is then divided and
ligated. The appendix is then tied at its base, excised and removed.
Most surgeons still opt to invaginate the appendix stump as a precautionary
measure against slippage of the stump ligature.
The rectum (Figs 17.2, 12.5)
• The rectum measures 10–15 cm in length. It commences in front of
the 3rd sacral vertebra as a continuation of the sigmoid colon and follows
the curve of the sacrum anteriorly. It turns backwards abruptly in
front of the coccyx to become the anal canal.
• The mucosa of the rectum is thrown into three horizontal folds that
project into the lumenathe valves of Houston.
• The rectum lacks haustrations. The teniae coli fan out over the rectum
to form anterior and posterior bands.
• The rectum is slightly dilated at its lower endathe ampulla, and is
supported laterally by the levator ani.
• Peritoneum covers the upper two-thirds of the rectum anteriorly but
only the upper third laterally. In the female it is reflected forwards onto
the uterus forming the recto-uterine pouch (pouch of Douglas). The
rectum is separated from anterior structures by a tough fascial sheet
athe rectovesical (Denonvilliers) fascia.
The anal canal (Fig. 17.2)
The anorectal junction is slung by the puborectalis component of levator
ani which pulls it forwards. The canal is approximately 4 cm long
and angled postero-inferiorly. Developmentally the midpoint of the
anal canal is represented by the dentate line. This is the site where the
proctodeum (ectoderm) meets endoderm. This developmental implication
is reflected by the following characteristics of the anal canal:
• The epithelium of the upper half of the anal canal is columnar. In contrast
the epithelium of the lower half of the anal canal is squamous. The
mucosa of the upper canal is thrown into vertical columns (of Morgagni).
At the bases of the columns are valve-like folds (valves of Ball).
The level of the valves is termed the dentate line.
• The blood supply to the upper anal canal (see Fig. 12.5) is from the
superior rectal artery (derived from the inferior mesenteric artery)
whereas the lower anal canal is supplied by the inferior rectal artery
(derived from the internal iliac artery). As mentioned previously, the
venous drainage follows suit and represents a site of porto-systemic
anastomosis (see p. 35).
• The upper anal canal is insensitive to pain as it is supplied by autonomic
nerves only. The lower anal canal is sensitive to pain as it is supplied
by somatic innervation (inferior rectal nerve).
• The lymphatics from the upper canal drain upwards along the superior
rectal vessels to the internal iliac nodes whereas lymph from the
lower anal canal drains to the inguinal nodes.