The =rectum= (Intestinum rectum) is the terminal part of the bowel; it
extends from the pelvic inlet to the anus.[95] Its length is about one
foot (ca. 30 cm.). Its direction may be straight or oblique. The first
or =peritoneal part= of the rectum is like the small colon, and is
attached by a continuation of the colic mesentery termed the
=mesorectum=. The second or =retroperitoneal part= forms a flask-shaped
dilatation termed the =ampulla recti=; it is attached to the surrounding
structures by connective tissue and muscular bands.
The first part of the rectum is related to the small colon and the
pelvic flexure of the great colon. It is frequently deflected to the
left by the latter. The second part of the rectum is related dorsally
and laterally to the pelvic wall. Ventrally the relations differ in the
two sexes. In the male they are the bladder, the terminal parts of the
vasa deferentia, the vesiculæ seminales, the prostate, the
bulbo-urethral (Cowper’s) glands, and the urethra. In the female they
are the uterus, vagina, and vulva.[96]
=Structure of the Large Intestine.=—The =serous coat= covers the
different parts in varying degrees. It does not cover (_a_) the opposed
surfaces of the cæcum and colon which are between the layers of the
cæco-colic fold and mesocolon; (_b_) the areas of attachment of the
cæcum and colon to the pancreas, right kidney, and sublumbar region;
(_c_) the second part of the rectum.
The =muscular coat= consists of longitudinal and circular fibers. The
bulk of the former is in the bands already described. Some of the
circular fibers pass from one part of the colon to another, where they
are attached to each other, forming the fibræ transversæ coli. The
muscular coat of the second part of the rectum presents special
features. The longitudinal layer of fibers is very thick and consists of
large bundles, loosely united. A large band, the =recto-coccygeus=, is
detached from it on either side, and passes upward and backward to be
inserted into the fourth and fifth coccygeal vertebræ.
The =submucous tissue= is abundant in the wall of the rectum, so that
the mucous membrane is loosely attached to the muscular coat, and forms
numerous folds when the bowel is empty.
The =mucous membrane= of the large intestine is thicker and darker in
color than that of the small intestine. It forms large crescentic or
semilunar folds corresponding to the external constrictions. It has no
villi, Brunner’s glands, or Peyer’s patches. The =intestinal glands= (of
Lieberkühn) are large and numerous. =Solitary glands= are also numerous,
especially at the apex of the cæcum and in the left dorsal part of the
colon.
=Blood-supply.=—Greater and lesser mesenteric and internal pudic
arteries. The veins go to the portal vein.
=Nerve-supply.=—Mesenteric plexus of the sympathetic nerves.
The =anus= is the terminal orifice of the alimentary canal. It is
situated below the root of the tail, where it forms a round projection,
with a central depression when contracted. It is covered externally by
an integument which is thin, hairless, and provided with numerous
sebaceous and sweat glands. The mucous lining is pale, glandless, and
covered with a thick, squamous, stratified epithelium.
There are three muscles of the anus.
1. The =sphincter ani internus= is merely a terminal thickening of the
circular coat of the bowel.
2. The =sphincter ani externus= is a broad band of striped muscle-fibers
outside the internal sphincter. Some fibers are attached to the
coccygeal fascia above, others to the perineal fascia below. Its action
is to close the anus.
3. The =retractor ani= (M. levator ani) is a flat muscle which lies
between the rectum and the sacro-sciatic ligament. It arises from the
superior ischiatic spine and the sacro-sciatic ligament, and ends under
the external sphincter. Its action is to reduce the partial prolapse
which the anus undergoes during defecation.
The suspensory ligament of the anus is a band of unstriped muscle which
arises from the first coccygeal vertebra, passes downward over the
retractor, and unites with its fellow below the anus. In the male it is
largely continued by the retractor penis muscle; in the female it blends
with the constrictor vulvæ. It may act as an accessory sphincter of the
anus.
=Blood-supply.=—Internal pudic artery.
=Nerve-supply.=—Hæmorrhoidal and perineal nerves (for the sphincter ani
externus and retractor ani).