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Large Intestine

The large intestine extends from the ileocaecal junction to anus. It is 1.5 meters long and is divided into caecum, ascending colon, right colic (hepatic) flexure, transverse colon, left colic (splenic) flexure, descending colon, sigmoid colon, the rectum and anal canal. Transverse colon is longest part (50 cm) and anal canal shortest (3.8cm).

Caecum ~ 6 cms. Sigmoid colon → 37.5 cm
Ascending colon ~ 12.5 cm Rectum → 12 cm
Transverse colon ~ 50 cm Anal canal → 3.8 cm
Descending colon ~ 25 cm  


Characteristic features of large intestine are: -

  1. 3 longitudinal bands formed by longitudinal muscle coat, called Taeniae coli.
  2. Sacculation or haustration
  3. Fat filled peritoneal pouches called appendices epiploicae. These are not found in appendix,caecum, and rectum.
  4. Greater part is fixed except for appendix, transverse colon and sigmoid colon.
    Peyer’s patches (present in small intestine) are not present.

Arterial supply

  1. The blood supply of colon is derived from the marginal artery of Drummond. It is a paracolic anastomotic artery formed by anastomosis between colic branches of superior mesenteric artery (ileocolic, right colic, middlecolic) and colic branches of inferior mesenteric artery (left colic and sigmoidal arteries). Terminal branches from marginal artery are distributed as long and short vessels vasa longa and vasa bravia.
  2. There are areas of colon with poor blood supply resulting from incomplete anastomosis of marginal arteries. These are watershed areas of colon and include:
    1. Splenic flexure (Griffith point): Watershed area between superior mesenteric artery and inferior mesenteric artery.
    2. Rectosigmoid junction (Sudeck's point): Watershed zone between inferior mesenteric artery and internaliliac artery.

Lymphatic drainage


Lymph from the large intestine passes through four sets of lymph nodes:-

  1. Epicolic Lymph nodes: Lying in the wall of gut.
  2. Paracolic nodes: On medial side of ascending and descending colon, and near mesocolic border of transverse and sigmoid colon.
  3. Intermediate nodes: On the main-branches of vessels.
  4. Terminal nodes, which are superior mesenteric and inferior mesenteric nodes (both are Preaortic nodes.)

Vermiform appendix

  1. It is a narrow worm-like tubular diverticulum which arises from the postero-medial wall of the caecum about 2 cm below ileocaecal function and is suspended by a peritoneal fold known as mesoappendix or appendicular mesentery. Length of appendix is 2-20 cm (average 9 cm). The appendix lies in the right iliac fossa. Base of the appendix is fixed, but the tip can point in any direction, according to which positions of appendix are classified: -
    1. Retrocaecal (12 0' clock) position: It is the commonest position(in 65% of individuals). ii) Pelvic (4 0' clock) position: It is second commonest position (in 30% of individuals).
    2. Paracolic (II 0' clock) position: In 20% of subjects.
    3. Midinguinal (6 0' clock) position: In 2% of subjects.
    4. Pre and post-ileal (30' clock) position: In 1 % and 0.4% subjects, respectively. vi) Promontoric(3 0' clock) position: In <I % of subjects.
  2. Appendix develops from midgutand therefore is supplied by superior mesenteric artery through appendicular branch of ileocolic artery.

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  1. Rectum is the most dilatable part of large intestine which is placed between the sigmoid colon (above) and the anal canal (below). It is 12 cm long. The upper part of rectum has same diameter as sigmoid colon, but the lower l/3rdis dilated to form rectal ampulla.
  2. The rectum is not straight but is curved in an anteroposterior direction and also from side to side. The three cardinal features of large intestine, i.e. sacculation, appendices epiploicae and taeniae are absent in the rectum, The upper two-third of rectum is cover with peritoneum (upper 1/3 in front and side, and middle 1/3 only in front), and lower 1/3 is devoid of peritoneum.

Mucosal folds


The mucous membrane of an empty rectum shows two types of fold, longitudinal and transverse. The longitudinal folds are temporary folds which are present in lower part of empty rectum and are obliterated on distension. Transverse or horizontal folds or Houston's valves (plica transversalis) are permanent and most prominent in distended position. There are:

  1. First (upper) fold: It projects from right wall.
  2. Second fold: It projects from left wall.
  3. Third (middle) fold (Valve of Houston): It is the largest and most constant. It projects from anterior and right walls. Valve of Houston disappear after mobilization of rectum but not by distension. Valve of Houston do not contain all muscle wall layers.

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Blood supply

  1. Major arterial supply of rectum is through superior rectal artery (inferior continuation of inferior mesenteric artery). Other arterial supply of rectum includes middle rectal arteries (branch of anterior division of internal iliac artery) and median sacral artery (branch of the aorta).
  2. Venous drainage is through superior rectal vein (drains into inferior mesenteric vein), middle rectal vein (drains into internal iliac vein) and median sacral vein (joins left common iliac vein).

Lymphatic drainage

  1. Upper half: To inferior mesenteric nodes (a group of Preaortic nodes) along superior rectal vessels after passing through perirectal and sigmoid nodes.
  2. Lower half: To internal iliac nodes along middle rectal vessels.

Nerve supply

  1. The rectum is supplied by sympathetic (L1, L2) fibers through inferior mesenteric plexus and parasympathetic (S2, S3, S4) by pelvic splanchnic nerve via inferior hypogastric plexus. Sympathetic nerves are inhibitory to rectal muscles and motor to the internal sphincter (prevent defecation). Parasympathetic nerves are motor to rectal muscles and inhibitory to internal sphincter (stimulate defecation).
  2. Pain fibers accompany both sympathetic and parasympathetic supply, while the sensation of distension is conveyed by parasympathetic afferents.

Supports of rectum


Supports of rectum include -

  1. Pelvic floor (levator ani)
  2. Fascia of Waldeyer: It attaches the lower part of rectal ampulla to the sacrum. It is formed by condensation of pelvic fascia behind the rectum and encloses the superior rectal vessels and lymphatics.
  3. Lateral ligaments of the rectum: It is formed by condensation of pelvic fascia and encloses middle rectal vessels, and branches of pelvic plexuses.
  4. Rectovesical fascia of Denonvilliers: It extends from rectum (behind) to the prostate and seminal vesicle in front.
  5. Pelvic peritoneum and related vascular pedicles.
  6. Perineal body with its muscles.



The anal canal is the terminal part of the alimentary canal. It begins at ano-rectal junction which is situated 2-3 cms in front and slightly below the tip of coccyx. From ano-rectal junction anal canal passes downwards and backwards through the pelvic diaphragm and opens at anal orifice (anus) which is situated in the cleft between the buttocks about 4 cm below and in front of the tip of coccyx. Sacculation and taeniae are absent in anal canal. The length of anal canal is 3.8 cm.


Interior of anal canal


The interior of anal canal is divided by pectinate line and Hilton's white line into three parts.

  1. Upper part (mucous part)
    It is upper 15 mm part of anal canal and extends upto pectinate line. It is lined by mucous membrane which is simple columnar. Mucosa shows 6-10 longitudinal folds called anal columns of Morgagni. Lower ends of these folds are united by short transverse folds, anal valves. Line of attachment of anal valve is known as pectinate line or dentate line. Slight depression above each anal valve is known as anal sinus.
  2. Middle part (transitionalzone or pecten)
    This area is 15 mm in extent and intervenes between pectinate (dentate) line and Hilton's white line. It is lined by non-keratinized stratified squamous epithelium without sebaceous and sweat glands.
  3. Lower part (cutaneous) part
    It is about 8 mm in extent and is lined by true skin, i.e. stratified squamous keratinizing epithelium with sebaceous and sweat glands.

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Nerve supply


Mucous membrane above pectinate line is supplied by autonomic nerves (sympathetic L1-L2 and parasympathetic: S2 S3 S4) and therefore is insensitive to pain. Part below the pectinate line is supplied by somatic nerves (inferior rectal nerve a branch of pudendal nerve) and therefore is sensitive to pain. Thus pain sensations are carried by pudendal nerve.


Lymphatic drainage


Pectinate line forms the water-shed line of anal canal. Area above pectinate line drains into internal iliac nodes and area below it drains into superficial inguinal nodes.


Blood supply

  1. Arterial supply of the area is above pectinate line is through superior rectal artery (continuation of inferior mesenteric artery) and area below pectinate line is supplied by inferior rectal artery.
  2. Venous drainage above the pectinate line is through superior rectal vein which drains into inferior mesenteric vein, a branch of splenic vein (portal system) and below the pectinate line is through inferior rectal vein which drains into internal pudendal vein (systemic circulation). Thus pectinate line is the level of portosystemic anastomosis and internal hemorrhoids develop just above pectinate line. Venous drainage occurs through ;-
    1. Internal rectal venous plexus (Hemorrhoidal plexus); - It lies in the submucosa of anal canal and drains mainly into superior rectal vein, but communicates freely with external plexus. The internal plexus is in the form of a series of dilated pouches and veins present in the 3,7 and 11 o'clock positions (as seen in lithotomy position) are large and constitute potential sites for primary internal piles (hemorrhoids).
    2. External rectal venous plexus; - It lies outside the muscular coat of rectum and anal canal. Lower part is drained by inferior rectal vein into internal iliac vein; the middle part by middle rectal vein into internal iliac vein; and upper part by superior rectal vein.
    3. Anal veins; - These are arranged radially around anal margin and communicate with internal plexus and inferior rectal veins. Rupture of these veins may cause perianal hematoma called external piles.

Anal sphincters


Two sphincters, internal and external, surround the anal canal.

  1. Internal sphincter (sphincter ani internus)
    It is an involuntary sphincter formed by thickening of circular muscle of lower part of rectum. It surrounds upper 3/4 of anal canal and lower end of internal sphincter corresponds with Hilton's line. Internal sphincter is supplied by sympathetic fibers through superior hypogastric plexus and by parasympathetic fibers from pelvic splanchnic nerves (S2 S3 S4). Internal sphincter remains in the state of tonic contraction most of time to maintain resting tone or pressure.
  2. External sphincter (sphincter ani externus)
    It is a voluntary sphincter and surrounds the entire length of anal canal. It is made up to striated muscle and is consists of three parts - Subcutaneous, superficial and deep. It is supplied by inferior rectal nerve and perineal branch of S4.

External sphincter is contributed by fibers from puborectalispart of levator ani muscle (in upper most part); superficial transverse perineal muscles anteriorly and anococcygeal raphe posteriorly (in upper third) and anococcygeal ligament (in middle third).

Piles (hemorrhoids)

  1. Internal piles or true piles are dilatation of internal rectal venous plexus. They occur above the pectinate line and are therefore painless. The primary piles occur in 3 o'clock (left lateral), 7 o 'clock (right posterior) and 11 o 'clock (right anterior) position. Piles in other positions are called secondary piles.
  2. External piles (False piles) occur below the pectinate line and are therefore painful as area below pectinate line is supplied by somatic nerve, (inferior rectal nerve, and a branch of pudendal nerve.

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