Caecum, Appendix and Colon Anatomy
The
large intestine is approximately 1.5 m long and comprises the caecum, appendix,
colon, rectum and anal canal. The caecum and appendix lie in the right iliac
fossa, while the colon runs a circuitous course (Figs 4.68–4.70) before descending into the pelvic cavity
to become continuous with the rectum. Descriptions of the rectum and anal canal
are given in Chapter 5.
The lumen of the intestine is relatively wide in the caecum and
ascending colon but narrows gradually as the colon is traced towards the
rectum.
The outer longitudinal muscle coat of the caecum and colon is
thickened to form three longitudinal bands, the taeniae coli (Fig. 4.71). Bulges
(haustrations) of the gut wall between the taeniae correspond with sacculations
on the mucosal surface. The peritoneal surface of the colon (but not of the
appendix or caecum) is characterized by numerous fat-filled tags, the omental
appendices (appendices epiploicae).
The ileum terminates by opening into the large intestine at a
slit-like or oval aperture, the ileal orifice (ileocaecal valve) (Fig. 4.72).
The caecum is the blind-ending portion of the large intestine
below the level of this orifice. The caecum and ascending colon are in direct
continuity and the three taeniae coli descend along the outer surface of the
caecum and converge on its posteromedial aspect at the root of the appendix.
The caecum usually lies in the right iliac fossa above the lateral
half of the inguinal ligament (Fig. 4.73). Anteriorly, it is related to the
abdominal wall, the greater omentum and coils of ileum; the iliacus and psoas
muscles lie posteriorly. The caecum may lie free in
the iliac fossa, completely surrounded by peritoneum. Alternatively, it may be
attached to the iliac fossa by peritoneal folds forming a retrocaecal fossa
(Fig. 4.71). Rarely, the caecum is entirely retroperitoneal. On the medial side
of the caecum, adjacent to the terminal ileum, small peritoneal folds may
enclose one or more ileocaecal recesses.
The worm-like appendix is attached to the posteromedial wall of
the caecum, where the taeniae coli converge (Fig. 4.71). It is a thick-walled
tube with a narrow lumen and, although variable in length,
usually measures approximately 10 cm. The surface marking of the root of the
appendix is relatively constant, lying one-third of the distance from the
anterior superior iliac spine to the umbilicus (Fig. 4.2).
The appendix usually possesses a mesentery, the mesoappendix (Fig. 4.72), which
is attached to the mesentery of the ileum and confers upon the appendix a
degree of mobility. Although it frequently lies behind the caecum (Fig. 4.74),
in front of the iliacus or psoas muscles, it occasionally descends into the
pelvis where its tip may lie adjacent to the bladder, the right ureter and, in
the female, the ovary or uterine tube (Fig. 4.73). Rarely, the appendix lies anterior
or posterior to the terminal ileum. It is a common site of infection
(appendicitis), which may spread to adjacent structures. Usually, the pain of
appendicitis arises initially from the gut wall and is periumbilical. When the
inflammation spreads to the parietal peritoneum the pain becomes localized to
the right iliac region.
Colon
The colon consists of ascending, transverse, descending and sigmoid
parts (Fig. 4.68). The ascending and descending parts are usually
retroperitoneal, while the transverse and sigmoid parts are suspended by
mesenteries. The colon possesses two acute angulations, the right (hepatic) and
left (splenic) flexures (Fig. 4.69).
The
ascending colon begins at the level of the ileal orifice and runs vertically
upwards. At the right colic flexure under the right lobe of the liver, it
becomes continuous with the transverse colon. Anterior to the ascending colon
lie the abdominal wall, loops of ileum and often the greater omentum. Its
posterior relations include the iliacus, transversus abdominis, quadratus
lumborum and the right kidney.
The transverse colon extends from the right to the left colic
flexure and is suspended by the transverse mesocolon. Typically, its middle
portion hangs downwards and crosses the umbilical region (Figs 4.69 &
4.70). At the left colic flexure just below the splenic hilum, it turns
inferiorly to become the descending colon. From right to left, the superior
relations of the transverse colon include the liver and gall bladder and the
greater curvature of the stomach. Posteriorly lie the descending duodenum, the
pancreas, loops of small intestine and the spleen. Anteriorly lie the greater
omentum and the abdominal wall. The greater omentum
attaches to both the transverse colon and the greater curvature of the stomach
(p. 160).
The descending colon runs from the left flexure to the left iliac fossa and
turns medially at the pelvic brim to continue as the sigmoid colon (Figs 4.68
& 4.76). The upper part of the descending colon is covered anteriorly by
coils of jejunum, while the lower part usually makes contact with the abdominal
wall, through which it is often palpable. Its posterior relations include the left
kidney and the psoas, quadratus lumborum and iliacus muscles. The sigmoid
(pelvic) colon begins at the pelvic brim and terminates in front of the third
sacral vertebra by joining the rectum. The sigmoid colon varies in length and
is mobile on its mesentery, the sigmoid mesocolon. Its proximal portion usually
runs to the right across the lower abdomen (Fig. 4.68) and is related
superiorly to loops of small intestine. The remainder of the sigmoid colon lies
in the pelvic cavity in contact with the upper surfaces of the pelvic organs. A
long pelvic mesocolon can provide mobility so that the colon may twist
(volvulus), causing ischaemia or colonic obstruction. Pockets of evaginated
colonic mucosa are common and may become infected (diverticulitis).
Blood supply
The blood supply of the caecum, appendix, ascending colon and most
of the transverse colon is provided by the superior mesenteric vessels. The
remainder of the colon is supplied by the inferior mesenteric vessels.
Branches of superior mesenteric vessels The
origin and course of the superior mesenteric artery are described on p. 177.
Its branches to the large intestine vary considerably but usually include the
middle colic, right colic and ileocolic arteries (Fig. 4.75).
The middle colic artery enters the transverse mesocolon and
divides into right and left branches, which supply the proximal two-thirds of
the transverse colon. The right colic artery reaches the ascending colon and
divides into ascending and descending branches, which supply the organ. The
ileocolic artery has anterior and posterior caecal branches and also supplies
the ascending colon and terminal ileum. The appendicular branch (Fig. 4.72)
descends behind the terminal ileum, enters the mesoappendix and runs near its
free border to the tip of the appendix. The arteries supplying the caecum and
colon anastomose, often forming a continuous marginal artery (Fig. 4.75).
Veins corresponding to the branches of the superior mesenteric
artery drain into the superior mesenteric vein (Figs 4.75 & 4.78).
Inferior mesenteric vessels
The inferior mesenteric artery arises from the anterior aspect of
the abdominal aorta 3 cm or 4 cm above the bifurcation, often overlapped by the
horizontal duodenum (Fig. 4.76). The artery runs retroperitoneally downwards
and to the left to reach the pelvic brim. Here it crosses the common iliac
vessels and continues into the pelvis as the superior rectal artery (p. 218).
The first branch of the inferior mesenteric artery, the left colic
artery, runs to the left and gives rise to ascending and descending branches.
The former supplies the distal third of the transverse colon and the left colic
flexure. The descending branch supplies the descending colon and the
commencement of the sigmoid colon. The inferior mesenteric artery gives rise to
several sigmoid arteries which reach the sigmoid colon via the mesocolon.
The branches of the inferior mesenteric artery are accompanied by
tributaries of the inferior mesenteric vein (Fig. 4.76). The course and
termination of this vessel are described with the portal venous system.
