Large Intestine
The large intestine extends from the
ileum to the anus. It is divided into the cecum, appendix, ascending colon,
transverse colon, descending colon, and sigmoid colon. The rectum and anal
canal are considered in the sections on the pelvis and perineum. The primary
function of the large intestine is the absorption of water and electrolytes and
the storage of undigested material until it can be expelled from the body as
feces.
Cecum
Location
and Description
The cecum is that part of the large
intestine that lies below the level of the junction of the ileum with the large
intestine). It is a blind-ended pouch that is situated in the right iliac fossa.
It is about 2.5 in. (6 cm) long and is completely covered with peritoneum. It
possesses a considerable amount of mobility, although it does not have a
mesentery.
Attached to its posteromedial surface is
the appendix. The presence of peritoneal folds in the vicinity of the cecum creates
the superior ileocecal, the inferior ileocecal, and the retrocecal recesses.
As in the colon, the longitudinal muscle
is restricted to three flat bands, the teniae
coli, which converge on the base of the appendix and provide for
it a complete longitudinal muscle coat. The cecum is often distended with gas
and can then be palpated through the anterior abdominal wall in the living
patient.
The terminal part of the ileum enters
the large intestine at the junction of the cecum with the ascending colon. The opening
is provided with two folds, or lips, which form the so-called ileocecal valve.
The appendix communicates with the
cavity of the cecum through an opening located below and behind the ileocecal
opening.
Relations
Anteriorly: Coils
of small intestine, sometimes part of the greater omentum, and the anterior
abdominal wall in the right iliac region
Posteriorly: The
psoas and the iliacus muscles, the femoral nerve, and the lateral cutaneous
nerve of the thigh. The appendix is commonly found behind the cecum.
Medially: The
appendix arises from the cecum on its medial side.
Blood
Supply
Arteries- Anterior
and posterior cecal arteries form the ileocolic artery, a branch of the
superior mesenteric artery
Veins- The
veins correspond to the arteries and drain into the superior mesenteric vein.
Nerve
Supply- Branches from the sympathetic and parasympathetic (vagus)
nerves form the superior mesenteric plexus.
Ileocecal Valve
A rudimentary structure, the ileocecal
valve consists of two horizontal folds of mucous membrane that project around the
orifice of the ileum. The valve plays little or no part in the prevention of
reflux of cecal contents into the ileum. The circular muscle of the lower end
of the ileum (called the ileocecal
sphincter by physiologists) serves as a sphincter and controls
the flow of contents from the ileum into the colon.
The smooth muscle tone is reflexly
increased when the cecum is distended; the hormone gastrin, which is produced by the stomach, causes
relaxation of the muscle tone.
Appendix
Location
and Description
The appendix is a narrow, muscular tube
containing a large amount of lymphoid tissue. It varies in length from 3 to 5
in. (8 to 13 cm). The base is attached to the posteromedial surface of the
cecum about 1 in. (2.5 cm) below the ileocecal junction. The remainder of the
appendix is free. It has a complete peritoneal covering, which is attached to
the mesentery of the small intestine by a short mesentery of its own, the mesoappendix. The mesoappendix contains
the appendicular vessels and nerves.
The appendix lies in the right iliac
fossa, and in relation to the anterior abdominal wall its base is situated one
third of the way up the line joining the right anterior superior iliac spine to
the umbilicus (McBurney’s point).
Inside the abdomen, the base of the
appendix is easily found by identifying the teniae coli of the cecum and
tracing them to the base of the appendix, where they converge to form a
continuous longitudinal muscle coat.
The tip of the appendix is subject to a
considerable range of movement and may be found in the following positions:
·
Hanging down into the pelvis against the
right pelvic wall
·
Coiled up behind the cecum,
·
Projecting upward along the lateral side
of the cecum
·
In front of or behind the terminal part
of the ileum. The first and second positions are the most common sites.
Blood
Supply
Arteries The
appendicular artery is a branch of the posterior cecal artery
Veins The appendicular
vein drains into the posterior cecal vein.
Nerve
Supply-The appendix is supplied by the sympathetic and
parasympathetic (vagus) nerves from the superior mesenteric plexus. Afferent
nerve fibers concerned with the conduction of visceral pain from the appendix
accompany the sympathetic nerves and enter the spinal cord at the level of the
10th thoracic segment.
Ascending Colon
Location
and Description
The ascending colon is about 5 in. (13
cm) long and lies in the right lower quadrant. It extends upward from the cecum
to the inferior surface of the right lobe of the liver, where it turns to the
left, forming the right colic flexure,
and becomes continuous with the transverse colon. The peritoneum
covers the front and the sides of the ascending colon, binding it to the
posterior abdominal wall.
Relations
Anteriorly: Coils
of small intestine, the greater omentum, and the anterior abdominal wall
Posteriorly: The
iliacus, the iliac crest, the quadrates lumborum, the origin of the transversus
abdominis muscle, and the lower pole of the right kidney. The iliohypogastric
and the ilioinguinal nerves cross behind it.
Blood
Supply
Arteries The
ileocolic and right colic branches of the superior mesenteric artery supply
this area.
Veins The veins correspond
to the arteries and drain into the superior mesenteric vein
Nerve
Supply- Sympathetic and parasympathetic (vagus) nerves from
the superior mesenteric plexus supply this area of the colon.
Transverse Colon
Location
and Description
The transverse colon is about 15 in. (38
cm) long and extends across the abdomen, occupying the umbilical region. It
begins at the right colic flexure below the right lobe of the liver and hangs
downward, suspended by the transverse mesocolon from the pancreas. It then
ascends to the left colic flexure below
the spleen. The left colic flexure is higher than the right colic flexure and
is suspended from the diaphragm by the phrenicocolic
ligament
Transverse mesocolon-
Also called mesentery of the transverse colon,
it suspends the transverse colon from the anterior border of the pancreas. The
mesentery is attached to the superior border of the transverse colon, and the
posterior layers of the greater omentum are attached to the inferior border.
Because of the length of the transverse mesocolon, the position of the
transverse colon is extremely variable and may sometimes reach down as far as
the pelvis.
Relations
Anteriorly: The
greater omentum and the anterior abdominal wall (umbilical and hypogastric
regions)
Posteriorly: The
second part of the duodenum, the head of the pancreas, and the coils of the
jejunum and the ileum.
Blood
Supply-
Arteries The
proximal two thirds are supplied by the middle colic artery, a branch of the
superior mesenteric artery. The distal third is supplied by the left colic artery,
a branch of the inferior mesenteric artery.
Veins The veins
correspond to the arteries and drain into the superior and inferior mesenteric
veins.
Nerve
Supply-The proximal two thirds are innervated by sympathetic
and vagal nerves through the superior mesenteric plexus; the distal third is
innervated by sympathetic and parasympathetic pelvic splanchnic nerves through
the inferior mesenteric plexus.
Descending Colon
Location
and Description
The descending colon is about 10 in. (25
cm) long and lies in the left upper and lower quadrants. It extends downward
from the left colic flexure, to the pelvic brim, where it becomes continuous
with the sigmoid colon. The peritoneum covers the front and the sides and binds
it to the posterior abdominal wall.
Relations
Anteriorly: Coils
of small intestine, the greater omentum, and the anterior abdominal wall
Posteriorly: The
lateral border of the left kidney, the origin of the transversus abdominis
muscle, the quadrates lumborum, the iliac crest, the iliacus, and the left
psoas. The iliohypogastric and the
ilioinguinal nerves, the lateral cutaneous nerve of the thigh, and the femoral nerve
also lie posteriorly.
Blood
Supply-
Arteries The
left colic and the sigmoid branches of the inferior mesenteric artery supply
this area.
Veins The veins
correspond to the arteries and drain into the inferior mesenteric vein.
Nerve
Supply
The nerve supply is the sympathetic and
parasympathetic pelvic splanchnic nerves through the inferior mesenteric plexus.
Differences
between the Small and Large Intestine
External Differences
1.
The small intestine (with the exception of the duodenum) is mobile, whereas the
ascending and descending parts of the colon are fixed.
2.
The caliber of the full small intestine is smaller than that of the filled
large intestine.
3.
The small intestine (with the exception of the duodenum) has a mesentery that
passes downward across the midline into the right iliac fossa.
4.
The longitudinal muscle of the small intestine forms a continuous layer around
the gut. In the large intestine (with the exception of the appendix), the
longitudinal muscle is collected into three bands, the teniae coli.
5.
The small intestine has no fatty tags attached to its wall.
6.
The large intestine has fatty tags, called the appendices epiploicae.
7. The wall of the small
intestine is smooth, whereas that of the large intestine is sacculated.
Internal Differences
1. The mucous membrane of
the small intestine has permanent folds, called plicae circulares, which are absent in the large intestine.
2. The mucous membrane of
the small intestine has villi, which are absent in the large intestine.
3. Aggregations of
lymphoid tissue called Peyer’s patches
are found in the mucous membrane of the small intestine; these are
absent in the large intestine.
Sigmoid
Colon
Location and Description
The sigmoid colon is 10 to 15 in. (25 to
38 cm) long and begins as a continuation of the descending colon in front of
the pelvic brim. Below, it becomes continuous with the rectum in front of the
3rd sacral vertebra. The sigmoid colon is mobile and hangs down into the pelvic
cavity in the form of a loop. The sigmoid colon is attached to the posterior
pelvic wall by the fan-shaped sigmoid
mesocolon.
Relations
Anteriorly: In
the male, the urinary bladder; in the female, the posterior surface of the
uterus and the upper part of the vagina
Posteriorly: The
rectum and the sacrum. The sigmoid colon is also related to the lower coils of
the terminal part of the ileum.
Blood Supply
Arteries-
Sigmoid
branches of the inferior mesenteric artery
Veins-
The
veins drain into the inferior mesenteric vein, which joins the portal venous
system.
Nerve Supply- The
sympathetic and parasympathetic nerves from the inferior hypogastric plexuses.
Rectum
Location and Description
The rectum is about 5 in. (13 cm) long
and begins in front of the third sacral vertebra as a continuation of the
sigmoid colon. It passes downward, following the curve of the sacrum and
coccyx, and ends in front of the tip of the coccyx by piercing the pelvic
diaphragm and becoming continuous with the anal canal.
The lower part of the rectum is dilated
to form the rectal ampulla. The
rectum deviates to the left, but it quickly returns to the median plane.
On lateral view, the rectum follows the
anterior concavity of the sacrum before bending downward and backward at its
junction with the anal canal. The puborectalis portion of the levator ani
muscles forms a sling at the junction of the rectum with the anal canal and
pulls this part of the bowel forward, producing the anorectal angle.
The peritoneum covers the
anterior and lateral surfaces of the first third of the rectum and only the
anterior surface of the middle third, leaving the lower third devoid of
peritoneum.
The muscular coat of the rectum
is arranged in the usual outer longitudinal and inner circular layers of smooth
muscle. The three teniae coli of the sigmoid colon, however, come together so
that the longitudinal fibers form a broad band on the anterior and posterior
surfaces of the rectum.
The mucous membrane of the rectum,
together with the circular muscle layer, forms two or three semicircular permanent
folds called the transverse folds of
the rectum they vary in position.
Relations
Posteriorly- The
rectum is in contact with the sacrum and coccyx; the piriformis, coccygeus, and
levatores ani muscles; the sacral plexus; and the sympathetic trunks.
Anteriorly-
In the male, the
upper two thirds of the rectum, which is covered by peritoneum, is related to
the sigmoid colon and coils of ileum that occupy the rectovesical pouch. The
lower third of the rectum, which is devoid of peritoneum, is related to the
posterior surface of the bladder, to the termination of the vas deferens and
the seminal vesicles on each side, and to the prostate.
In the female, the
upper two thirds of the rectum, which is covered by peritoneum, is related to
the sigmoid colon and coils of ileum that occupy the rectouterine pouch (pouch of
Douglas). The lower third of the rectum, which is devoid of peritoneum, is
related to the posterior surface of the vagina.
Blood Supply-
Arteries-The
superior, middle, and inferior rectal arteries supply the rectum. The superior rectal artery is a direct
continuation of the inferior mesenteric artery and is the chief artery
supplying the mucous membrane. It enters the pelvis by descending in the root
of the sigmoid mesocolon and divides into right and left branches, which pierce
the muscular coat and supply the mucous membrane. They anastomose with one another
and with the middle and inferior rectal arteries.
The middle rectal artery is a small branch of the internal iliac
artery and is distributed mainly to the muscular coat. The inferior rectal artery is a branch of
the internal pudendal artery in the perineum. It anastomoses with the middle
rectal artery at the anorectal junction.
Veins-The
veins of the rectum correspond to the arteries. The superior rectal vein is a tributary of the portal
circulation and drains into the inferior mesenteric vein. The middle and inferior rectal veins drain into the internal iliac and internal
pudendal veins, respectively. The union between the rectal veins forms important
portal–systemic anastomoses
Nerve Supply- The
nerve supply is from the sympathetic and parasympathetic nerves from the
inferior hypogastric plexuses. The rectum is sensitive only to stretch.
Anal
Canal
Location and Description
The anal canal is about 1.5 in. (4 cm)
long and passes downward and backward from the rectal ampulla to the anus
Except during defecation, its lateral walls are kept in opposition by the
levatores ani muscles and the anal sphincters.
Relations
Posteriorly:
The anococcygeal body, which
is a mass of fibrous tissue lying between the anal canal and the coccyx.
Laterally: The
fat-filled ischiorectal fossae.
Anteriorly: In
the male, the perineal body, the urogenital diaphragm, the membranous part of
the urethra, and the bulb of the penis. In the female, the perineal body, the
urogenital diaphragm, and the lower part of the vagina.
Structure
The mucous membrane of the upper half
of the anal canal is derived from hindgut ectoderm. It has the following important
anatomic features:
1. It is lined by
columnar epithelium.
2. It is thrown into
vertical folds called anal columns, which
are joined together at their lower ends by small semi lunar folds called anal valves (remains of
proctodeal membrane)
3. The nerve supply is the
same as that for the rectal mucosa and is derived from the autonomic
hypogastric plexuses. It is sensitive only to stretch.
4. The arterial supply is
that of the hindgut—namely, the superior rectal artery, a branch of the
inferior mesenteric artery. The venous drainage is mainly by the superior
rectal vein, a tributary of the inferior mesenteric vein, and the portal vein.
5. The lymphatic drainage
is mainly upward along the superior rectal artery to the pararectal nodes and then
eventually to the inferior mesenteric nodes.
The mucous membrane of the lower half
of the anal canal is derived from ectoderm of the proctodeum. It has the
following important features:
1. It is lined by
stratified squamous epithelium, which gradually merges at the anus with the
perianal epidermis.
2. There are no anal columns
3. The nerve supply is from the somatic
inferior rectal nerve; it is thus sensitive to pain, temperature, touch, and pressure.
4. The arterial supply is the inferior
rectal artery, a branch of the internal pudendal artery. The venous drainage is
by the inferior rectal vein, a tributary of the internal pudendal vein, which
drains into the internal iliac vein.
5. The lymph drainage is downward to the
medial group of superficial inguinal nodes.
The
pectinate line indicates the level where the upper half of the anal
canal joins the lower half.
Muscle
Coat- As in the upper
parts of the intestinal tract, it is divided into an outer longitudinal and an
inner circular layer of smooth muscle.
Anal Sphincters
The
anal canal has two sphincters-
·
internal
sphincter- involuntary
·
external
sphincter- voluntary
The
internal sphincter is formed from a thickening of the smooth muscle of
the circular coat at the upper end of the anal canal. The internal sphincter is
enclosed by a sheath of striped muscle that forms the voluntary external sphincter.
The external sphincter can be
divided into three parts:
1. A subcutaneous part,
which encircles the lower end of the anal canal and has no bony attachments
2. A superficial part, which
is attached to the coccyx behind and the perineal body in front
3. A deep part, which
encircles the upper end of the anal canal and has no bony attachments.
The puborectalis fibers of the
two levatores ani muscles blend with the deep part of the external sphincter.
The puborectalis fibers of the two sides form a sling, which is attached in
front to the pubic bones and passes around the junction of the rectum and the
anal canal, pulling the two forward at an acute angle.
The longitudinal smooth muscle of the
anal canal is continuous above with that of the rectum. It forms a continuous coat
around the anal canal and descends in the interval between the internal and
external anal sphincters.
Some of the longitudinal fibers are
attached to the mucous membrane of the anal canal, whereas others pass
laterally into the ischiorectal fossa or are attached to the perianal skin.
At the junction of the rectum and anal
canal, the internal sphincter, the deep part of the external sphincter and the
puborectalis muscles form a distinct ring, called the anorectal ring, which
can be felt on rectal examination.
Blood Supply
Arteries-
The
superior artery supplies the upper half and the inferior artery supplies the
lower half.
Veins-
The
upper half is drained by the superior rectal vein into the inferior mesenteric
vein, and the lower half is drained by the inferior rectal vein into the
internal pudendal vein.
Nerve Supply-The
mucous membrane of the upper half is sensitive to stretch and is innervated by
sensory fibers that ascend through the hypogastric plexuses. The lower half is
sensitive to pain, temperature, touch, and pressure and is innervated by the
inferior rectal nerves.
The involuntary internal sphincter is
supplied by sympathetic fibers from the inferior hypogastric plexuses. The
voluntary external sphincter is supplied by the inferior rectal nerve, a branch
of the pudendal nerve and the perineal branch of the fourth sacral nerve.
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