Large intestine

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 (see below). 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 & LYMPH DRAINAGE
&NERVE 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.
• Lymph Drainage
• The lymph vessels pass through several mesenteric nodes and
finally reach the superior mesenteric nodes.
• 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
• 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.

POSITION OF THE APPENDIX
• Common Positions of the Tip of the Appendix The tip
of the appendix is subject to a considerable range
of movement and may be found in the following
positions:
(a) hanging down into the pelvis against the right
pelvic wall, (b) coiled up behind the cecum, (c)
projecting upward along the lateral side of the
cecum, and (d) in front of or behind the terminal part
of the ileum. The first and second positions are the
most common sites.

BLOOD SUPPLY & LYMPH DRAINAGE &
NERVE SUPPLY
Blood Supply
• Arteries The appendicular artery is a branch of the posterior
cecal artery.
• Veins The appendicular vein drains into the posterior cecal
vein.
Lymph Drainage
• The lymph vessels drain into one or two nodes lying in the
mesoappendix and then eventually into the superior
mesenteric nodes.
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
• 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 lobeof 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 quadratus 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 & LYMPHATIC
DRAINAGE & NERVE SUPPLY
Blood Supply
• Arteries The ileocolic and right colic branches of the superior
mesenteric artery supply this area.
superior mesenteric vein.
Lymph Drainage
• The lymph vessels drain into lymph nodes lying along the course of
the colic blood vessels and ultimately reach the superior
mesenteric nodes.
Nerve Supply
• Sympathetic and parasympathetic (vagus) nerves from the
superior mesenteric plexus supply this area of the colon.

TRANSVERSE COLON
• 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.The transverse mesocolon, or mesentery
of the transverse colon, 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 .
superior and inferior mesenteric veins.
Lymph Drainage
• The proximal two thirds drain into the colic nodes and then
into the superior mesenteric nodes; the distal third drains into
the colic nodes and then into the inferior mesenteric nodes.
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
• 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 quadratus 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.
inferior mesenteric vein.
Lymph Drainage
• Lymph drains into the colic lymph nodes and the inferior
mesenteric nodes around the origin of the inferior mesenteric
artery.
Nerve Supply
• The nerve supply is the sympathetic and parasympathetic pelvic
splanchnic nerves through the inferior mesenteric plexus.

SIGMOID COLON
• 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.
Lymph Drainage
• The lymph drains into nodes along the course of the
sigmoid arteries; from these nodes, the lymph travels to
the inferior mesenteric nodes.
Nerve Supply
• The sympathetic and parasympathetic nerves from the
inferior hypogastric plexuses.

RECTUM
• The rectum is about 5 in. (13 cm) long and begins in front of the third
sacral vertebra as a continuation of the sigmoidcolon. 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.

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
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 an important portal–systemic
anastomosis).

LYMPHATIC DRAINAGE & NERVE
SUPPLY
Lymph Drainage
• The lymph vessels of the rectum drain first into the pararectal nodes
and then into inferior mesenteric nodes.Lymph vessels from the
lower part of the rectum follow the middle rectal artery to the
internal iliac nodes.
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
• 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
apposition 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 (Fig. 8.5).
• ■■ 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 entoderm. It has the following
important anatomic features:
• ■■ It is lined by columnar epithelium.
• ■■ It is thrown into vertical folds called anal columns, which
are joined together at their lower ends by small semilunar
folds called anal valves (remains of proctodeal membrane).
• ■■ 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.
• ■■ 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

STRUCTURE
by the superior rectal vein, a tributary of the inferior mesenteric vein,
and the portal vein.
• ■■ 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:
• ■■ It is lined by stratified squamous epithelium, which gradually
merges at the anus with the perianal epidermis.
• ■■ There are no anal columns.
• ■■ The nerve supply is from the somatic inferior rectal nerve; it is thus
sensitive to pain, temperature, touch, and pressure.
• ■■ 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.
• ■■ 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 .

STRUCTURE
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 an involuntary internal sphincter and
a voluntary external sphincter. 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:
• ■■ A subcutaneous part, which encircles the lower end of the anal
canal and has no bony attachments
• ■■ A superficial part, which is attached to the coccyx behind and
the perineal body in front

STRUCTURE
• ■■ 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 & LYMPH DRAINAGE &
NERVE SUPPLY
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.
Lymph Drainage
• The upper half of the anal canal drains into the pararectal nodes
and then the inferior mesenteric nodes. The lower half drains into
the medial group of superficial inguinal nodes .
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

Large intestine

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