Anal Canal

ANAL  CANAL

• Anal canal is the terminal part of alimentary tube.
• It begins at the ano-rectal junction, which is situated about 2 – 3 cm in front and below the tip of coccyx.
• The canal passes downwards and backwards through the pelvic diaphragm.
• Below it opens into the anal orifice which is situated about 4 cm below and in front of the tip of coccyx.

RELATIONS

In Front :-

• Perineal body
• In male - bulb of penis & spongy urethra
• In female - lower part of posterior wall of vagina

Behind :-

• Ano-coccygeal raphe
• Fibro-fatty tissue

On Each Side :-

• Ischio-rectal fossa & its contents

INTERIOR OF ANAL CANAL

• Interior of the canal is divided by pectinate line & Hilton’s white line into 3 areas –
• Upper, intermediate & lower parts

Upper Area :-

• It is about 15 mm in extent.
• It is limited below by pectinate line.
• It is lined by semi-transparent mucous membrane with simple columnar epithelium.
• Internal rectal venous plexus lies outside the mucous membrane & makes the area plum red in colour.

Upper area presents following features –

Anal columns (Columns of Morgagni)

• These are permanent mucous folds
• Longitudinal in direction
• 6 – 10 in number
• Each column is formed by reduplication of mucous membrane.
• They contain radicles of superior rectal vessels.

Anal valves (Valves of Ball)

• These are crescentic mucous folds connect the lower ends of anal columns.
• Their free margins are directed upwards.
• Their upper surface is lined by simple columnar epithelium & lower surface by stratified epithelium.
• The position of valves is indicated by pectinate or dentate line.

Anal papillae

• These are epithelial processes projecting from the free margin of anal valves.

Anal sinuses

• These are recesses above the valves between the anal columns.

Anal glands

• Floor of the sinuses receive ducts of tubular anal glands.

Intermediate Area (Area of Pecten or Transitional zone) :-

• This area is 15 mm in extent.
• It intervenes between the pectinate line and Hilton’s line.
• It is lined by non-keratinised stratified squamous epithelium.
• The area is surrounded by internal rectal venous plexus which gives a bluish pink colour.
• It is richly supplied by vessels and somatic spinal nerves.

Lower Area (Anal verge) :-

• It is about 8 mm in extent.
• It is lined by true skin.
• The corrugation of perianal skin is due to the attachment of fibro-elastic septa formed by longitudinal muscles of rectum and levator ani.
• Such septa form corrugator cutis ani muscle.

Pectinate Line :-

• It forms the muco-cutaneous junction of anal canal and corresponds with position of anal valves.
• It is situated at the middle of internal sphincter muscle.

Importance –

• It divides the anal canal into upper and lower areas which differ in their development, arterial supply, venous drainage, lymphatic drainage & nerve supply.
• Development- Upper area is developed from endodermal cloaca & lower area from ectodermal proctodeum.
• Arterial supply- Upper area is supplied by superior rectal artery & lower area by inferior rectal artery.
• Venous drainage- Upper area is drained into portal system via superior rectal vein & lower area into systemic veins via inferior rectal veins.

• Lymphatic drainage- Upper area is drained into internal iliac nodes & lower area into superficial inguinal lymph nodes (lymph channels form water-shed line).

• Nerve supply- Upper area is supplied by autonomic nerves (superior hypogastric plexus, pelvic splanchnic nerves) & lower area by somatic spinal nerves via inferior rectal nerves.

Hilton’s Line :-

• It is a colour contrast between bluish pink area above and black skin below.
• The line is represented by anal inter-sphincteric groove.

Importance –

• It indicates lower end of internal sphincter muscle.
• Here anal intermuscular septum is attached.

SPHINCTERS OF ANAL CANAL

• Internal & external anal sphincters.

Internal Anal Sphincter :-

• It is an involuntary sphincter.
• Formed by thickening of circular muscle of rectum.
• It surrounds the upper 3/4^th of anal canal.
• Lower end of internal sphincter corresponds with Hilton’s line and its middle corresponds with pectinate line.
• Internally it is separated from mucous membrane by internal venous plexus.
• Externally it is separated from external sphincter muscle by a conjoint sheath.

Nerve supply –

• Sympathetic fibres derived from superior hypogastric plexus.
• Parasympathetic fibres derived from pelvic splanchnic nerves.

External Anal Sphincter :-

• It is voluntary sphincter & surrounds the entire length of anal canal.
• It consists of three parts – Subcutaneous, superficial & deep.

Subcutaneous part –

• It presents a flattened band around the anus.
• It is separated from perineal skin by external venous plexus.

Superficial part –

• It is elliptical in shape.
• Arises from the tip of coccyx & anococcygeal raphe.
• It is inserted into perineal body.

Deep part –

• It is annular in shape, surrounds the ano-rectal junction.
• It is inserted into perineal body & continuous with superficial transverse perineal muscles.

Nerve supply –

• External sphincter is supplied by inferior rectal branch of pudendal nerve & perineal branch of fourth sacral nerve.

Anorectal Ring :-

• It is a muscular ring at the anorectal junction
• Formed by upper part of internal sphincter, deep part of external sphincter & puborectalis part of levator ani muscles.

APPLIED ASPECTS

• 85% of ano-rectal diseases start primarily at the pectinate line.
• Ischio-rectal abscess, when communicates with the anal canal, usually opens at or below the Hilton’s line.
• Sometimes, foreign bodies may get impacted in the anal sinuses and produce infection of the canal.

Anal Fistulae :-

• They are fibrous tracts communicating with two surfaces – ano-rectal mucosa & the skin.
• They start from infected anal glands in the inter-sphincteric region between the internal and external anal sphincters.

Anal Fissure :-

• Sometimes, anal valve may get torn during the passage of hard faeces & produces a painful condition known as anal fissure.
• A sentinel pile is a tag formed by a ruptured anal valve.

Haemorrhoids / Piles :-

• In portal obstruction superior rectal veins passing through the anal columns in left lateral, right posterior and right anterior positions are distended much earlier than the other veins & form the sites of primary internal piles.
• They are classified into internal & external haemorrhoids.

Internal Haemorrhoids -

• Distended & varicose veins above the pectinate line.
• Rectal bleeding with bright red blood is the main symptom of internal haemorrhoids.

External Haemorrhoids -

• Distended & varicose veins below the pectinate line.
• External haemorrhoids are covered by skin and may be painful due to thrombosis or rupture of peri-anal veins.
• Sometimes, during straining the prolapse of haemorrhoids takes place through the anal orifice and the prolapsed mass may be often strangulated.