The large intestine

Beginning at the ileocaecal valve, the greater part of
the large intestine (Fig. 3.15) consists of the ascending,
transverse, descending and sigmoid colon. The caecum
is a cul-de-sac that projects from the proximal
part of the ascending colon and is the major site of
bacterial vitamin synthesis in the large intestine. The
vermiform appendix, which extends from the caecum,
is vestigial in the human. The length of the large
intestine in the adult human is usually 1.5–1.8 m. The
calibre is 7.5 cm at its commencement and diminishes
gradually until it reaches a minimum of 2.5 cm at the
rectosigmoid junction. The calibre then increases to
form the rectal ampulla, which narrows abruptly at
the tonically contracted anal canal.
The well-developed muscularis externa of the large
intestine comprises an outer longitudinal and inner
circular layer of smooth muscle. Most of the longitudinal
muscle fi bres aggregate into three equidistantly
placed bundles known as taeniae. The combined
contractions of the circular and longitudinal muscles
cause the unstimulated portion of the large intestine
to bulge outward into bag-like sacs called haustra. The
mixing movements arising from haustral contractions
gradually expose all the colonic contents to the
intestinal epithelium. The slow but persistent haustral
contractions gradually propel the fl uid contents
in the caecum and ascending colon anal-ward. From
the beginning of the transverse colon to the sigmoid
colon, the haustral contractions are taken over by
peristalsis-like mass movements. The submucosa and
muscularis mucosa of the large intestine are similar to
those of the small intestine. The mucosa of the entire
large intestine is devoid of villi. The intestinal glands
are larger and more numerous than those in the small
intestine, and contain a greater abundance of mucusproducing
goblet cells.