888
UNIT 4
Maintenance of the Body
23
(a)
Right colic
(hepatic) flexure
Transverse colon
Superior
mesenteric
artery
Haustrum
Ascending colon
IIeum
IIeocecal valve
Appendix
Cecum
Left colic
(splenic) flexure
Transverse mesocolon
Epiploic appendages
Descending colon
Tenia coli
Sigmoid colon
Cut edge of mesentery
External anal sphincter
Rectum
Anal canal
(b)
Rectal valve
Rectum
Anal canal
Levator ani muscle
Anus
Anal sinuses
Anal columns
Internal anal
sphincter
External anal
sphincter
Hemorrhoidal
veins
Pectinate line
Figure 23.29
Gross anatomy of the large intestine.
(a)
Diagrammatic view.
(b)
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
anal columns
.
Anal sinuses
, recesses
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
pectinate line
. Superior
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
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