D R . N A B I L K H O U R I , M D . P H . D

LARGE INTESTINE

ANATOMY AND HISTOLOGY OF THE GIT HOLLOW

ORGANS III

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.

LARGE INTESTINE

•Has three unique features:

•Teniae coli – three bands of longitudinal smooth muscle in its muscularis

•Haustra – pocketlike sacs caused by the tone of the teniae coli

•Epiploic appendages – fat-filled pouches of visceral peritoneum

•Is subdivided into the cecum, appendix, colon, rectum, and anal canal

•The saclike cecum:

•Lies below the ileocecal valve in the right iliac fossa

•Contains a wormlike vermiform appendix

ASCENDING COLON

Location and Description

• The ascending colon 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 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

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 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 terminal part of the ileum enters the large intestine

at the junction of the cecum with the ascending colon.

• This 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.

ILEO-CECAL VALVE

• 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.

APPENDIX

Location and Description

• The appendix is a narrow, muscular tube containing a large amount of lymphoid tissue.

• It varies in length from 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 line joining the right anterior superior iliac spine to the umbilicus (McBurney’s point).

The glands are lined with simple

columnar epithelium and a high

number of mucin producing goblet

cells.

The lamina propria typically contains

lymphocytes that partly obscure the

underlying muscularis

Mucosae.

The submucosa is almost fully

occupied by lymphoid tissue mainly

arranged in lymphatic nodules.

The center of the lymphoid nodules

stain lighter and are termed germinal

centers.

The germinal center contains the larger

dividing lymphoblasts, similar to the

arrangement in lymph nodes.

The outer portions of the submucosa

harbor larger vessels and have less

dense infiltrates of immune cells.

TRANSVERSE COLON

Location and Description • The transverse colon is about 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

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

DESCENDING COLON

Location and Description • The descending colon is about 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..

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.

SIGMOID COLON

Location and Description • The sigmoid colon is 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.

LOCATION AND DESCRIPTION RECTUM

• The rectum is about 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 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 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 .

ANO-RECTAL JUNCTION

ANAL CANAL

Location and Description • The anal canal is about 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 arterial supply is that of the hindgut—namely, the

superior rectal artery, a branch of the inferior mesenteric

artery. The venous drainage is mainly

VALVES AND SPHINCTERS OF THE RECTUM AND ANUS

•Three valves of the rectum stop feces from being

passed with gas

•The anus has two sphincters:

•Internal anal sphincter composed of smooth muscle

•External anal sphincter composed of skeletal

muscle

•These sphincters are closed except during defecation

HISTOLOGY

•Colon mucosa is simple columnar epithelium

except in the anal canal

•Has numerous deep crypts lined with goblet cells

•Anal canal mucosa is stratified squamous

epithelium

•Anal sinuses exude mucus and compress feces

•Superficial venous plexuses are associated with the

anal canal

•Inflammation of these veins results in itchy

varicosities called hemorrhoids

LARGE INTESTINE

Mucosa

folded

No plecae circulares

two types of cells

Goblet Mucus-secreting

Absorptive

Glands packed straight tubular

Crypts extend to the MM

LARGE INTESTINE

SMALL VS LARGE INTESTINE

Small intestine Large intestine

Villi 1. Crypts shallow 2. Goblet cells less

1. Absence of

villi

2. Crypts

deeper, More Goblet cells

Longitudinal

muscle coat

of muscularis externa

1. Uniformly thick 1. Three bands of Taenia coli