Maintenance of the Body
Cut edge of mesentery
External anal sphincter
Levator ani muscle
Gross anatomy of the large intestine.
Structure of the anal canal.
enzymes. However, its mucosa is thicker, its abundant crypts are
deeper, and the crypts contain tremendous numbers of goblet
cells. Mucus produced by goblet cells eases the passage of feces
and protects the intestinal wall from irritating acids and gases
released by resident bacteria.
Te mucosa of the anal canal, a stratiﬁed squamous epithe-
lium, merges with the true skin surrounding the anus and is quite
diﬀerent from the mucosa in the rest of the colon, reﬂecting the
greater abrasion that this region receives. Superiorly, it hangs in
long ridges or folds called
between the anal columns, exude mucus when compressed by
feces, which aids in emptying the anal canal (Figure 23.29b).
Te horizontal, tooth-shaped line that parallels the inferior
margins of the anal sinuses is called the
to this line, visceral sensory ﬁbers innervate the mucosa, which
is relatively insensitive to pain. Te area inferior to the pectinate
line is very sensitive to pain, a reﬂection of the somatic sensory
ﬁbers serving it.
±wo superﬁcial venous plexuses are associated with the anal
canal, one with the anal columns and the other with the anus