The colon extends from the end of the ileum to the rectum. The
cecum, ascending colon, hepatic flexure, and proximal transverse
colon comprise the right colon. The distal transverse
colon, splenic flexure, descending colon, sigmoid colon, and rectosigmoid
comprise the left colon (Figure 30–1).
The ascending and descending portions are fixed to the retroperitoneum,
and the transverse colon and sigmoid colon are suspended in the
peritoneal cavity by their mesocolons. The caliber of the lumen
is greatest at the cecum and diminishes distally. The wall of the
colon has four layers: mucosa, submucosa, muscularis, and serosa
(Figure 30–2). The muscularis propria
consists of an inner circular layer and an outer longitudinal layer.
The longitudinal muscle completely encircles the colon in a very
thin layer, and at three points around the circumference it is gathered
into thick bands called taeniae coli. Sacculations (haustra) are
the result of shortening of the colon by the taeniae and contractions
of the circular muscle. The haustra are not fixed anatomic structures
and may be observed to move longitudinally. There are fatty appendages
(the appendices epiploicae) on the serosal surface. The wall of
the colon is so thin that it becomes markedly distended when obstructed.
The large intestine: anatomic divisions and blood supply.
The veins are shown in black. The insert shows the usual configuration
of the colon.
Cross section of colon. The longitudinal muscle encircles
the colon but is thickened in the region of the taeniae coli.
The length of the rectum varies from 12 to 16 cm,
and it is dependent on an individual’s body habitus. The
taeniae coli spread out at the rectosigmoid junction and are not
apparent distal to that area. The upper rectum is invested by peritoneum
anteriorly and laterally, but posteriorly it is retroperitoneal
up to the junction with the sigmoid colon. The anterior peritoneal
reflection dips low into the pelvis and may be as low as 5–8
cm above the anal verge. The anterior peritoneal reflection lies
behind the bladder in males and behind the uterus (the rectouterine
pouch of Douglas) in females. Tumor masses or abscesses in this
location are readily palpated on digital rectal or pelvic examination.
The rectum is normally distensible and serves a function as a capacitance
organ. When its capacity to distend is lost or impaired by surgery
or disease, fecal urgency and frequency are noted.
The rectal valves of Houston are prominent spirally arranged
mucosal folds within the rectum. Less than half of people have the
so-called normal three valves, two on the left and one on the right.
The valves are at variable distances from the anal verge in different
individuals. Normally, the valves appear thin, with sharp edges,
but they become thickened ...