The Large Intestine

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Jejunum and ileum | Main Anatomy Index | The liver

Last updated 30 March 2006

Large Intestine

The abdominal part of the large intestine is about 1.5 m long.
It consists of the caecum, vermiform appendix, colon (ascending, transverse, and sigmoid), and rectum.
Three parts of the large intestine form a frame for the small intestine.

The large intestine can easily be distinguished from the small intestine by:

Taeniae coli, three thickened bands of longitudinal muscle.
The sacculations of its walls between the taeniae, called haustra.
Appendices epiploicae (omental appendages), the small pouches of omentum filled with fat.

The Caecum and Appendix

The Caecum

The sac-like caecum (L. caecus, blind) is the 1^st part of the large intestine and is obviously continuous with the ascending colon.
The ileum opens into its superior part at the ileocaecal junction.
About 2.5 cm inferior to this, the vermiform appendix opens into its medial aspect.

The caecum is a broad blind pouch and is 5 to 7 cm in length.
It is located in the right lower quadrant, where it lies in the iliac fossa, inferior to the ascending colon.

Usually the caecum is completely covered by peritoneum and can be freely lifted. It, however, does not have a mesentery.
Sometimes the peritoneal covering of the caecum is absent posteriorly and it is bound to varying extents by the posterior abdominal wall.

It is frequently attached by peritoneal caecal folds to the iliac fossa laterally and medially.
This forms a cul-de-sac of the peritoneal cavity, called the retrocolic recess.
This recess is often deep enough to admit a digit.
In 64% of people, the appendix lies in it.

The ileum enters the caecum obliquely, and partly invaginates into it, forming lips superior and inferior to the ileocaecal orifice.
These lips of the ileocaecal valve meet medially and laterally to form ridges, called the frenula of the ileocaecal valve.
However, the circular muscle is poorly developed in them and the ileocaecal valve has little sphincteric action.

The Vermiform Appendix

This is a narrow, worm-shaped blind tube (L vermis, worm + forma, form).
It is variable in length, averaging 8 cm.
It joins the caecum about 2.5 cm inferior to the ileocaecal junction and is relatively longer in infants and children than in adults.

The appendix has its own short triangular mesentery, called the mesoappendix.
This suspends it from the mesentery of the terminal ileum.

The position of the body of the appendix is variable: retrocaecal or retrocolic (65%), pelvic (31%), subcaecal (2.3%) and rarely anterior or posterior to the terminal ileum.

The base of the appendix is fairly constant and usually lies deep at the junction of the lateral and middle 1/3 of the line joining the ASIS and the umbilicus (McBurney's point).
The three taeniae coli of the caecum converge at the base of the appendix and form a complete outer longitudinal coat for it.

Arterial Supply of the Caecum and Appendix

The caecum is supplied by the ileocolic artery (--> anterior and posterior caecal arteries), a branch of the superior mesenteric artery.

The appendix is supplied by the appendicular artery (branch of the antrior caecal artery), a branch of the ileocolic artery.
It descends posterior to the terminal part of the ileum and enters the mesoappendix.

Venous Drainage of the Caecum and Appendix

The ileocolic vein, a tributary of the superior mesenteric vein, drains the blood of the caecum and appendix.

Lymphatic Drainage of the Caecum and Appendix

Lymph vessels from the caecum and appendix pass to the lymph nodes in the mesoappendix and to the ileocolic lymph nodes that lie along the ileocolic artery.
Efferent lymph nodes pass to the superior mesenteric lymph nodes.

Innervation of the Caecum and Vermiform Appendix

The nerves of the caecum and appendix are derived from the coeliac and superior mesenteric ganglia.

The Ascending Colon

The ascending colon (G. kolos, large intestine) various from 12 to 20 cm in length.
It ascends on the right side of the abdominal cavity from the caecum to the right lobe of the liver.
Here it turns left at the right colic (hepatic) flexure.

It usually has no mesentery and lies retroperitoneally along the right side of the posterior abdominal wall (some 25% of people have a short mesentery).
The ascending colon is covered by peritoneum anteriorly and on its side, which attaches it to the posterior abdominal wall.

The ascending colon is separated from the muscles of the posterior abdominal wall by the kidneys and inferior by the nerves of the posterior abdominal wall (ilioinguinal and iliohypogastric).
It is usually separated from the anterior abdominal wall by coils of small intestine and the greater omentum.

On the lateral side of the ascending colon, the peritoneum forms a trench or groove called the right paracolic gutter.
The depth of this groove depends on how much has the ascending colon contains.

The Arterial Supply of the Ascending Colon

The ascending colon and right colic flexure are supplied by the ileocolic and right colic arteries, branches of the superior mesenteric arteries.

Venous Drainage of the Ascending Colon

The ileocolic and right colic veins, tributaries of the superior mesenteric vein, drain the blood of the ascending colon.

Lymphatic Drainage of the Ascending Colon

The lymph vessels of the ascending colon pass to the paracolic and epicolic lymph nodes and from them to the superior mesenteric lymph nodes.

Innervation of Ascending Colon

These nerves to the ascending colon are derived from the coeliac and superior mesenteric ganglia.

The Transverse Colon

This section hangs down as a loop to a variable extent.
The transverse colon, about 45 cm in length, is the largest and most mobile part of the large intestine.
It crosses the abdomen from the right colic flexure to the left colic flexure, where it bends inferiorly to become the descending colon.

The left colic flexure lies on the inferior part of the left kidney and is attached to the diaphragm by the phrenicocolic ligament.
The left colic flexure is more superior and more posterior than the right colic flexure.

Between these 2 colic flexures, the transverse colon is freely movable and forms a loop that is directed inferiorly and anteriorly.
The transverse colon has a mesentery known as the transverse mesocolon, which is connected to the inferior border of the pancreas and to the greater omentum that covers it anteriorly.

Because it is freely movable, the transverse colon is extremely variable in position.
It may be at the level of the transpyloric plane or it may extend inferiorly as far as the pelvic brim.

Arterial Supply of the Transverse Colon

The transverse colon is mainly supplied by the middle colic artery, a branch of the superior mesenteric artery.
It also receives blood from the left and right colic arteries.
The left colic artery is a branch of the inferior mesenteric artery.

Venous Drainage of the Transverse Colon

Venous drainage is via the superior mesenteric vein.

Lymphatic Drainage of the Transverse Colon

Lymph from the transverse colon passes to the lymph nodes that lie along the middle colic artery.
The superior mesenteric lymph nodes receive lymph vessels from these nodes.

Innervation of the Transverse Colon

The nerves that follow the right and left colic arteries are derived from the superior mesenteric plexus.
They transmit sympathetic and vagal nerve fibres.

The nerves that follow the left colic artery are derived from the inferior mesenteric plexus.

The Descending Colon

This part of the large intestine is 22 to 30 cm in length.
It descends from the left colic flexure into the left iliac fossa, where it is continuous with the sigmoid colon.
As it descends, the colon passes anterior to the lateral border of the left kidney and the transversus abdominis muscle and quadratus lumborum.

The calibre of the descending colon is considerably smaller than that of the ascending colon.

It is usually has no mesentery and lies retroperitoneally along the left side of the posterior abdominal wall.
Its posterior surface is attached to the posterior abdominal wall like the ascending colon.
In some people (33%), the descending colon has a mesentery.

The descending colon is related to the diaphragm superiorly and the quadratus lumborum muscle.
The iliohypogastric and ilioinguinal nerves intervene between it and this muscle.

Arterial Supply of the Descending Colon

The descending colon is supplied by the left colic and superior sigmoid arteries, branches of the inferior mesenteric artery.

Venous Drainage of the Descending Colon

The descending colon is drained by the inferior mesenteric vein.

Lymphatic Drainage of the Descending Colon

The lymph vessels of the descending colon pass to the intermediate colic lymph nodes, along left colic artery.
From them, the lymph passes to the inferior mesenteric lymph nodes (around the inferior mesenteric artery).

Innervation of the Descending Colon

It receives its sympathetic supply from the lumbar part of the sympathetic trunk and the superior hypogastric plexus by means of plexuses on the branches of the inferior mesenteric artery.

The parasympathetic supply is derived from the pelvic splanchnic nerves.

The Sigmoid Colon

The sigmoid colon forms a sinuous, S-shaped loop of variable length (usually 40 cm).
This is portion of large intestine between the descending colon and rectum.

It extends from the pelvic brim to the 3^rd segment of the sacrum, where it joins the rectum.

The termination of the taeniae coli indicates the beginning of rectum.
It usually has a long mesentery, the sigmoid mesocolon and thus has considerable freedom of movement.

The sigmoid colon usually occupies the rectovesical pouch in males and the rectouterine pouch in females.
The root of its mesentery has a V-shaped attachment, superiorly along the external iliac vessels and inferiorly from the bifurcation of the common iliac vessels to the anterior aspect of the sacrum.
Posterior to the apex of the mesentery (i.e., retroperitoneally) lies the left ureter and the division of the left common iliac artery.

The appendices epiploicae (omental appendages) are very long in the sigmoid colon.

Faeces are usually stored in the sigmoid colon before defecation.

Posterior to the sigmoid colon is the left external iliac vessels, the left sacral plexus and the left piriformis muscle.

Arterial Supply of the Sigmoid Colon

There are 2 to 3 sigmoid arteries and these are branches of the inferior mesenteric artery.
The most superior sigmoid artery anastomoses with the descending branch of the left colic artery.

Venous Drainage of the Sigmoid Colon

The inferior mesenteric vein drains the blood from the sigmoid colon.

Lymphatic Drainage of the Sigmoid Colon

Lymph passes to the intermediate colic lymph nodes on the branches of the left colic arteries, and from them to the inferior mesenteric lymph nodes.

Innervation of the Sigmoid Colon

It receives its sympathetic supply from the lumbar part of the sympathetic trunk and the superior hypogastric plexus by means of plexuses on the branches of the inferior mesenteric artery.

The parasympathetic supply is derived from the pelvic splanchnic nerves.

The Rectum

This is continuous with the sigmoid colon at the midpiece of the sacrum.
It has a length of about 12 cm.
It descends along the sacro-coccygeal concavity as the sacral flexure.

It eventually joins the anal canal at the anorectal junction, 2 to 3 cm in front of the coccygeal tip.
The bend at this point is known as the perineal flexure of the rectum.

The rectum is covered by peritoneum on its anterior surface and sides in the upper 1/3, anterior surface only in the middle 1/3 and is not covered in the lower 1/3.

The lower part of the rectum is dilated as the rectal ampulla.
The upper part has 3 transverse rectal folds (upper and lower on the left, the middle on the right).

The upper part of the rectum above the middle fold may contain faeces, but the lower part only contains faeces in chronic constipation or during the call to defecate.

The rectum has several important relations:

	Anterior	Posterior	Laterally
Males	Base of bladder Seminal vesicles Rectovesical pouch Ileum Sigmoid colon Ductus deferens Terminal ureter Prostate gland	S3-S5 vertebrae Coccyx Median sacral artery and vein Ganglion impar Superior rectal artery and vein	Sigmoid colon Ileum Pelvic plexuses Coccygeus muscle Levator ani muscle
Females	Uterus Vagina Rectouterine pouch Ileum Sigmoid colon	As for male	As for male

The Anal Canal

This is about 4 cm long in adults.

Posterior to it lies the anococcygeal ligament, separating it from the tip of the coccyx.
Anteriorly, it is separated from the lower vagina or membranous urethra and penile bulb by the perineal body.
Laterally, it is related to the ischiorectal fossa.

The upper half of the anal canal is lined by mucosa, which is plum red due to the internal rectal venous plexus.
The lower half is lined with stratified squamous non-keratinising epithelium (continuous with the skin of the anus).

In this lower half, there are 6 to 7 anal columns.
Each column contains a terminal branch of the superior rectal artery and vein, these being largest at the 3, 7 and 11 o'clock positions.
Enlargement of the venous terminal branches and anastomoses give rise to internal haemorrhoids.

The lower ends of the columns are linked by anal valves, above each of which is an anal sinus.
The anal valves together are known as the pectinate line, which is situated opposite the internal anal sphincter.

The anal canal extends below the pectinate line as the pecten, which is bluish in colour.
The pecten ends inferiorly at the "white line" of Hilton.

Anal Musculature

The internal anal sphincter (smooth muscle) surrounds the anorectal junction and is a thickening of the rectal circular muscle. It ends at the white line.

The external anal sphincter (striated muscle) surrounds the whole anal canal and consists of three parts: subcutaneous, superficial and deep, going from inferior to superior.

At the anorectal junction, the pubococcygeal fibres of the levator ani fuse with the longitudinal smooth muscle coat of the rectum to form a conjoint longitudinal coat for the anal canal, which lies between the internal and external anal sphincters.