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

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.

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 cecalvein.

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

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 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.

• Veins The veins correspond to the arteries and drain into the

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

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.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 & LYMPHATIC DRAINAGE & NERVE SUPPLY

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.

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

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 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 & LYMPHATIC DRAINAGE & NERVE SUPPLY

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.

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

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 & LYMPHATIC DRAINAGE & NERVE SUPPLY

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

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 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

• ArteriesThe 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.

• VeinsThe 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

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 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 hypogastricplexuses. 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