Anal canal is the terminal part of large intestine. It is situated below the level of pelvic diaphragm, lies in anal triangle of perineum between right and left ischiorectal fossae.

Length, extent and direction:

Anal canal is 3.8 to 4 cm. in length and extends from anorectal junction to anus and directed downwards and forwards. Though it is only 3.8 cm. long it is of greatest surgical importance because of its role in the mechanism of rectal continence and because it is prone to harbour certain diseases, for these reason its anatomy and closely related levator ani muscle require to be considered in detail.

In normal living subject the anal canal is completely collapsed owing to tonic contracture of the anal sphincter and the anal orifice, it is represented by an anterioposterior slit situated in midline. Posteriorly the canal is related to coccyx with a certain amount of fibrous fatty and muscle lining intervening known as “Anococcygeal” ligament. Laterally there is ischiorectal fossa on either sides which is ladened with fat and inferior haemorrhoidal vessel and nerve which crosses it to enter the wall of the canal. 34 Anteriorly in male the canal is related to the (1) central point of perineum i.e. the perineal body (2) Bulb of the urethra and posterior border of urogenital diaphragm (triangular ligament) containing the membranous urethra.

In Female the canal is related in front to (1) Perineal body and (2) lowest part of posterior vaginal wall. Anal canal musculature:- The anal wall is surrounded by complex of anal sphincters.

1) Internal sphincter:

This is involuntary muscle and thickened muscle coat of rectum, commence where the rectum, pass through pelvic diaphragm and ends at the anal orifice. It surrounds the upper 3/4th of anal canal i.e. 3 cm. long and ends at intersphincteric groove. The fibres are pearly white and runs transversally.

2) Conjoint longitudinal muscle:

It lies between the external and internal sphincter and is formed by fusion of the puborectalis with longitudinal muscle coat of rectum and anorectal junction. At lower level it becomes fibroelastic and spread out fanwise piercing the subcutaneous part of external sphincter and attaches to skin around the anus. Most lateral of these form the perianal fascia.

3) External sphincter:

It is striated muscle covering whole length of the anal canal and at lower end placed little below the internal sphincter and has three parts viz. subcutaneous, superficial and deep. 

i) Subcutaneous:

Lies below level of internal sphincter and surrounds lower part of anal canal. It is 15 mm. broad and has no bone attachment.

ii) Superficial:

It is elliptical in shape and arises from posterior surface of the terminal segment of coccyx and the anococcygeal ligament or raphe. The fibres surround the lower part of the internal sphincter and are inserted into perineal body.

iii) Deep part:

Surrounds the upper part of internal sphincter and fused with puborectalis it has no bone attachment. Internal of anal canal It can be divided into three parts

a) Upper part about 15 mm. long
b) Middle part of about 15 mm. long and
c) Lower part of about 8 mm. long

I) Upper mucosal part:

The mucosa is thrown into 8-14 longitudinal folds known as anal columns or column of Morgagni. Lower end of each column are united to each other by short transverse fold of mucous membrane and is called Anal valves. Above each valve there is depression in mucosa which is called as anal sinus. The anal valves together form a transverse line that run all round the anal canal and is known as pectinate line, it is situated just opposite the internal sphincter. The pectinate line is also known as Dentate line because of serrated fringe produced by the valves. 36 Pecten = Cock's comb (Latin ) Dentate = Toothed (Latin )

II) Middle part or Transitional zone of Pecten:

It is about 15 mm. and lined by mucous membrane but anal columns are absent. It has bluish appearance because of dense venous plexus that lies between it and muscle coat. The mucosa is mobile than in upper part of anal canal, it is devoid of sweat glands.

III) Lower cutaneous part:

It is about 8 mm. long and is lined by true skin containing sweat and sebaceous gland.

The Dentate line or Pectinate line:

It is the most important landmark both pathologically and surgically, it represents:

1) The site of fusion of the proctoderm and post allantoic gut.

2) The position of the anal membrane remnants of which frequently may be seen as anal papillae situated on the free margin of the anal valves.

Anatomical and Surgical importance of the Dentate line:

1) It forms the embryological watershed between visceral structure above and somatic structure below.

2) The mucosa above the line has autonomic nerve supply and is thus insensitive to cutting and pricking where as the skin and mucosa below is supplied by the inferior rectal branch of the pudendal nerve and is actually sensitive to these stimulii.

3) The anal glands open into the anal sinuses above the anal valve at this level. Infection in an anal gland may lead to an anal abscess which may extend into ischiorectal space and perianal space. 37

4) In the finer control of continence stimulation of nerve endings in the region of the dentate line may initiate reflex or voluantry changes on sphincter tone.

The Dentate line separates:

Above Below Cubical epithelium Squamous epithelium Autonomic nerve Spinal nerve (Pain insensitive) (Very much pain sensitive) i.e. Pudendal nerve Portal venous system Systemic venous system

The anal valve of Ball:

The anal valves of Ball are a series of transversally placed semilunar folds linking the columns of Morgagni, they lie along and actually constitute the wavieness of the dentate line.

Crypts of Morgagni:

These are small pockets between inferior extremities of the colums of Morgagni about 8 to 14 in number most of them are situated at posterior side and each open into anal glands by a narrow duct called anal duct, this duct bifurcate and pass to enter the internal sphincter muscle where there is ampulla. Infection of an anal gland can give rise to an abscess.

Anorectal ring:

It is a muscular ring present at anorectal junction and is formed by fusion of upper end of external and internal sphincter and puborectalis it is more marked posteriorly and laterally than anteriorly. Surgical division of this ring results in rectal incontinence. 

Arterial supply:

1) The part above pectinate line is supplied by superior rectal artery.
2) Below pectinate line it is supplied by inferior rectal artery.

Venous drainage:

1) The internal rectal venous plexus or haemorrhoidal plexus lies in submucosa of the anal canal, it drains into superior rectal vein but communicates freely with external plexus i.e. middle and inferior rectal vein and thus is an important site of communication on between portal and systemic veins. Veins present in the three anal columns situated at 3,7 and 11o clock position as seen in the lithotomy position and is site for formation of primary internal piles

2) The external rectal venous plexus lies outside the muscular coat of the rectum and anal canal and communicates freely with internal plexus and is drained by internal rectal vein into pudendal vein.

3) The middle rectal vein drains into internal iliac vein.

4) The anal veins are arranged radially around the anal margin, they communicate with internal rectal plexus and inferior rectal vein. Excessive straining during defecation may rupture one of these veins subcutaneous perianal haematoma known as external piles.

Nerve supply:

1) Above the pectinate line the anal canal is surrounded by autonomic nerve both sympathetic (inferior hypogastric plexus 4 to 12) and parasympathetic (pelvic splanchanic S 2, 3 and 4) nerves.

2) Below the pectinate line it is supplied by somatic (inferior rectal S 2,3 and 4) nerve. 

3) The internal sphincter is contracted by sympathetic and relaxed by parasympathetic nerve.

4) The external sphincter is supplied by inferior rectal and perineal branch of S4 nerve. Pelvic diaphragm:- It supports the rectum and other pelvic organ and prevents prolapsed of pelvic organ, it is formed by levator ani and coccygeous muscle.

I) The levator ani:

It forms the pelvic diaphragm supporting pelvic viscera. The structures passing through pelvic diaphragm lies within the sling of puborectalis muscle. It arises in continuity from pelvic bone in front and thickened obturator fascia and ischial spine, it is inserted into coccyx and anocccygeal ligament posteriorly. II) Coccygeous muscle:- It forms posterior part of pelvic floor; it?s under surface being continuous with sacro coccygeal ligament.

PHYISIOLOGY OF ANO RECTAL CANAL

Mechanism of defecation Defecation:

Defecation is an act of emptying the distal colon from the spleenic flexure through the anal orifice into exterior, which is a reflex process. The mechanism of defecation plays an important role in the development of haemorrhoids, if there is any alteration in the process of normal defecation there are more chances of 40 development of haemorrhoids. The main function of the rectum and anal canal is to expel the faeces which is present in the terminal part of alimentary canal viz. the descending colon. Defecation is a complex reflex mechanism which is under voluntary control of cerebral cortex at least in the ordinary condition of life, usually the rectum is empty in normal individual and contain faeces in cases of chronic constipation the urge of defecation occur after the stimulus to the initiation of the distension of rectum. It is likely that a summation of impulse is necessary to achive consciousness of a certain level of filling of rectum together with conditional reflex at the habitual time of the day.

Apart from the cerebral cortex there is a center in lumbo sacral region of spinal cord. The reflex centre for defecation is located in hypothalamus, lower lumbar and upper sacral segment of spinal cord and ganglionic plexus of the gut. The reflex is initiated by rise in intra luminal pressure pressure of about 20 to 25 cm. of water in rectum containing pressure receptors which not only detects increase of pressure but also differentiate whether the increase in pressure is due to gas, liquid or solid substance. A factor of prime importance in beginning of the act is the assumption of the squatting position which straightens out the angulation between the rectum and anal canal which facilitates emptying of the rectum. The pelvic floor descends and the physical forces in the anal canal are overcome by intra abdominal pressure. After the main mass of the faeces has passed through the anal canal muscles regain activity and finally discharge the stool.