Anal Canal

 What are the boundaries of Perineum?

Perineum is the diamond-shaped lowest region of trunk below the pelvic diaphragm. 

The skeletal and ligamentous boundaries of perineum correspond to those of pelvic outlet.

• Anteriorly: Lower border of pubic symphysis.
• Anterolaterally: Ischiopubic rami.
• Laterally: Ischial tuberosities.
• Posterolaterally: Sacrotuberous ligaments.
• Posteriorly: Tip of coccyx.

 What are the subdivisions of perineum and their contents?

An imaginary transverse line joining the anterior ends of ischial tuberosities divides the Perineum into two triangles:

• Urogenital triangle anteriorly: Mainly contains external genitalia, part of urethra and  superficial and deep perineal pouches with their contents.
• Anal triangle posteriorly: Mainly contains anal canal in the center and ischiorectal fossae on either side.

Describe the extent, location of anal canal.

Extent of Anal canal 

• Anal canal is the  terminal  part of  large intestine.
• It lis located in the center of the anal triangle of perineum below the pelvic diaphragm.
• It is 3-4cm long and is directed downwards and backwards from the rectum.
• Anus is its external opening.
• It is devoid of sacculations, tenea coli and appendices epiploicae.

What are the relations of anal canal?

• Anteriorly: perineal body
• Posteriorly: anococcygel ligament.
• Laterally:  Ischiorectal fossae. 

 What are parts of anal canal ?

• Interior of anal canal is divided into upper and lower parts by the pectinate line. The lower part is further divided into upper and lower regions by Hilton’s line.
• Pectinate line is the site of attachment of the anal membrane in fetus.

The upper and lower parts of anal canal  are different in development , blood and nerve supply, and  lymphatic drainage.

What are the differences between the upper and lower parts of anal canal?

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  Describe the internal features of different  parts of anal canal.

Upper part of anal canal

• It is 15mm long.

• It extends from anorectal junction to pectinate line.

• It is lined by simple columnar epithelium.

• It is reddish in colour and shows following features: 

• Anal columns/Columns of Morgagni: These are 6-10 permanent vertical mucosal folds. Each fold contains terminal radicles of superior rectal vessels.
• Anal valves : Are semilunar shaped transverse mucosal folds which connect the lower end of the anal columns. Free margins of the valves are directed upwards. The wavy pectinate or dentate line lies at this level.
• Anal sinuses:Are the vertical recesses between the anal columns and above the anal valves.

 Upper region of Lower Part of Anal canal (Pecten)

• Is 15mm long and extends from pectinate line to Hilton’s line.  The white line of Hilton demarcates mucocutaneous junction and corresponds to the intersphincteric groove ( lower end of internal sphincter muscle).
• It is lined by stratified squamous non-keratinized epithelium.
• It .is bluis in colour due to the presence of venous plexus underneath.

Lower Region of Lower Part of Anal Canal

• It is 8mm long and lies below Hilton’s line.
• Is lined by skin (stratified squamos keratined epithelium) containing sebaceous and sweat glands.
• It is black in clour.

Describe briefly the sphinters and the longitudinal coat of muscle of anal canal.

There are two sphincters of anal canal:

• Internal sphincter
• External sphincter

Internal anal sphincter

• Is formed by thickening of circular muscle coat of anal canal.
• It surrounds the upper 2/3rd of anal canal.
• It ends at the level of Holton’s line.
• Is made  up of smooth muscle and is under control of autonomic nerves and hence is involuntary.
• Is surrounded by deep and superficial parts of external anal sphincter.

External  anal sphincter

• It surrounds the whole length of anal canal.
• It is made up of skeletal muscle and is innervated by somatic nerve inferior rectal nerve) and therefore has voluntary control.
• It consists of  three parts
  • deep
  • superficial
  • subcutaneous
• Deep part of external anal sphincter:
  • Surrounds the upper part of anal canal.
  • Has no bony attacment.
  • Puborectalis part of levator ani blends with this part of external anal sphincter.
•  Superficial part of external anal sphincter
  •  It surrounds the lower part of internal anal sphincter.
  • It arises from the tip of the coccyx and anococcygeal ligament and is inserted into the perineal body.
  • Is elliptical in shape.
• Subcutaneous part of External anal sphincter
  • It lies below the level of internal anal sphincter.
  • It has no bony attachment.
  • It is traversed by fibroblastic septa derived from the conjoint longitudinal muscle coat of anal canal.

• Longitudinal muscle of the anal canal
  • Longitudinal muscle of the anal canal  separates the internal anal sphincter from the external anal sphincter.
  • It Fuses with part of puborectalis muscle to form the conjoint longitudinal sheet.
  • It becomes progressively fibroelastic and breaks into number of fibrous strands which pass through the subcutaneous part of external anal sphincter and attach to the skin around the anus.

 Describe in brief the arterial supply of Anal canal.

 Anal canal is supplied by the following two arteries:

•  Superior rectal artery::Is a branch of inferior mesenteric artery. Its branches form submucosal plexus which extends as terminal radicals in the anal columns.It supplies the mucosa of the anal canal above the pectinate line.
• Inferior rectal artery: Is a branch of internal pudendal
• artery. It supplies the mucosa of the anal canal below the pectinate line.

 Describe the venous drainage of anal canal.

• Above the pectinate line: Is drained by the internal venous plexus into the superior rectal vein which drains ultimately into the portal circulation.
• Below the pectinate line: Is drained by inferior rectal vein, a tributary of systemic vein.

 Applied Aspects