Anus

Surgically, the anal canal extends from the anal verge to the top of the anorectal ring (the point where external sphincter transitions to puborectalis, at the pelvic floor), which is about 4cm.

Anatomists would define is the anal verge to the dentate line (point of transition from visceral afferent to somatic afferent innervation), which is about 2cm

Embryology

Traditional teaching was that the lower third is derived from proctodaeum (ectodermal depression), and upper two-thirds from the hindgut. The dentate line was said to mark the division between these two structures, and the valves themselves were thought to be remnants of the proctodeal membrane, which separated the two structures. Therefore, the part below the dentate line is supplied by branches of the internal pudendal artery.
According to Jamieson's, this may not be completely true. Perhaps the whole of the anal transitional zone is the true endoderm-ectoderm boundary, which has subsequently enlarged. The border becomes much less defined with age, and stratified squamous epithelium often occurs proximal to the pectinate line.

Structure

Last part of the alimentary tract
Tube of muscle - all circular fibres, in two parts, which seem to act as a unit
Internal sphincter
- Visceral/smooth muscle, autonomically innervated, about 3cm long
- Thickened downward continuation of the inner circular muscle of the rectum (muscularis propria), which usually extends about 3/4 of the length of the anal canal
- Forms the intersphincteric groove where it terminates, 1cm below the pectinate line and about 1cm proximal to the distal edge of the external sphincter.
- Contributes 50-85% of the resting tone to anal canal
- Separated from the external sphincter by the longitudinal muscle
- Innervation
  - Not completely clear whether SNS or PNS causes relaxation
  - SNS from the hypogastric nerves derived from L5
  - PNS from pelvic splanchnic nerves
  - Rectoanal inhibitory reflex - internal sphincter relaxation in response to rectal distension. Maybe predominantly an intramural reflex.
The longitudinal muscle
- A downwards projection of smooth muscle extending down between the internal and external sphincters, partly formed from longitudinal coat of rectum
- Some fibres radiate through external sphincter to pecten region and skin, leading to the puckering we see
External sphincter
- Skeletal muscle
- Funnel-shaped structure made up of the pelvic floor muscles enveloping the distal rectum and anus
  - Traditionally divided into subcutaneous, superficial and deep parts, but this is not a particularly useful differentiation
  - Superficial - elliptical in shape, and attaches posteriorly to coccyx
  - Deep - blends intimately with puborectalis
- Blends with the puborectalis part of levator ani at its upper end (except in the midline at the front, where there are no levator ani fibres). The area where these blend is termed the anorectal ring, and is palpable on rectal examination.
- See 'perineum' topic for pelvic floor muscles
- External sphincter can be controlled by both autonomic and voluntary contractions
  - Reflex contractions in response to increased intra-abdominal pressure or rectal distension
  - Can be subject to muscular fatigue, with maximal voluntary contraction sustained for only 30-60 seconds
- Innervation
  - Supply from the inferior rectal branch of the pudendal nerve (S2, S3) and the perineal branch of the fourth sacral nerve
  - Crossover of fibres - unilateral damage does not abolish tonic discharge
  - Stimulation causes contraction of the external sphincter, which produces the squeeze pressure
  - Childbirth or chronic straining may stretch the nerves, causing a pudendal neuropathy resulting in faecal incontinence
Anal columns/Columns of Morgagni:
- Vertical folds in the mucosa of the anus
- The valves at the lower limits of the anal columns constitute the pectinate/dentate line, which is situated opposite the middle of the internal sphincter
- Mid-anal canal
- 6-10 of them
- Frequently contain a terminal branch of superior rectal artery and vein, supplemented to a variable degree by middle and inferior rectal vessels
- Dilated submucosal veins in upper anal canal for an internal rectal ('haemorrhoidal') venous plexus. Most prominent at 3, 7 and 9 o'clock as 'anal cushions'. These cushions help to seal the canal and prevent leakage.
Haemorrhoidal cushions
- The distal canal features a thickened layer of discontinuous tissue - the haemorrhoidal cushions, typically found 3, 7 and 11 o'clock
- These cushions receive blood supply from six haemorrhoidal arteries distributed among the circumference of the distal rectum and anus, and blood drains to superior, middle and inferior haemorrhoidal vessels, which allows for a portosystemic shunt
- Therefore, haemorrhoidal bleeding is arterial rather than venous
Anal glands/ducts
- Usually 5-6 glands
- Each gland drains via a duct into the base of a sinus created by an anal valve
- Not evenly located, mainly drain into the posterior sinuses
- Unclear whether there is much of a secretory function
- Some confined to submucosa, but two-thirds have one or more branches which enter the internal sphincter, and 50% of those pass across the internal sphincter completely to reach the intersphincteric longitudinal muscle layer
Pecten band
- Archaic term for a so-called circumferential band of fibrous tissue situated between the skin of the lower part of the anal canal, in the region of pecten and sphincter musculatrue
- Was said to be associated with fissure and haemorrhoids
- No such fibrous tissue was found on histological examination - this probably represents the rounded inferior border of the internal sphincter
Hilton's white line
- Another archaic term, avoid
- Was said to be a line marking intersphincteric groove, but most evidence says it does not exist

Adjacent structures

Anococcygeal ligament
- Musculotendinous structure running between external sphincter and coccyx
- Division of this structure causes descent of the anal canal and lowering of the posterior part of the anal triangle, but doesn't otherwise interfere with defecation

Adjacent spaces

Superficial post-anal space
- Posterior to anal canal
- Anterior to coccyx
- Superficial to ano-coccygeal ligament
Deep post-anal space (of Courtney)
- Deep to superficial post-anal space
- Deep to ano-coccygeal ligament
- Below levator ani
- Continuous laterally with ischio-anal space, which can lead to horseshoe abscesses
Inter-sphincteric space
- Between internal and external sphincters
- Continuous with supra-levator space - sepsis can track
Ischioanal/ischiorectal fossa
- Paired wedge-shaped spaces filled with fat lateral to the anal canal. The anococcygeal body separates the two fossae low-down, but they communicate through loose connective tissue behind the anal canal higher up (allows horseshoe spread of infection).
- Boundaries
  - Medial wall - anal canal and levator ani muscles
  - Lateral wall - ischial tuberosity (below), obturator internus (above)
  - Apex - where the medial and lateral walls meet (levator ani is attached to its tendinous origin over the obturator fascia)
  - Base - skin
  - Anterior wall - perineal body and urogenital diaphragm
  - Posterior wall - sacrotuberous ligament, overlapped by gluteus maximus
- Contents
  - Ischioanal fat pad - allows for distension of the anal canal during defecation, and the vagina during delivery
  - Pudendal (Alcock's) canal - connective tissue sheath in the lower lateral wall of the fossa, containing the pudendal nerve and internal pudendal vessels
  - Inferior rectal branches of the pudendal nerve and internal pudendal vessels - arch upwards as they run transversely across the fossa towards the anal canal
  - Posterior scrotal (labial) nerves and vessels (from the pudendals) - at the front of the fossa
  - Perineal branch of S4 - at the back of the fossa
  - Perforating cutaneous nerve

Vasculature

See 'colon/rectum' topic

Nerve supply

Internal sphincter. Contraction from sympathetic fibres from the pelvic plexus (L5). Relaxation from PNS (S2-S4).
External sphincter: Supplied by inferior rectal branch of the pudendal nerve (S2-3) and by the perineal branch of S4
Loss of unilateral pudendal nerve function will not lead to loss of function, but bilateral S3 loss will lead to incontinence
If unilateral S1-S3 roots remain intact, patients should have control of the anal sphincters
Sensation from the rectal branch of the pudendal nerve, which is also thought to play a role in continence
- Organised nerve endings include Meissner corpuscles (touch), Krause bulbs (temperature sensation), Golgi-Mazzoni bodies (pressure), and genital corpuscles (friction)

Lymphatic drainage

Above dentate line drains upwards to join lymphatics of rectum - internal iliac nodes
Pecten zone is a watershed - mostly follows perianal skin to superficial inguinal nodes, but there is some communication across this area
Below dentate line passes via the inferior haemorrhoidal system to superficial inguinal group

Microscopic/mucosal anatomy

Proximal part (at level of levator ani) lined by columnar epithelium
- Appears pleated by longitudinal folds, called the columns of Morgagni
- Small pocket or crypt at the base of most columns that communicates with the anal glands
- The 6-12 anal glands secrete lubricating fluid to assist with defecation, and are mostly found posteriorly
- The anal gland ducts traverse the submucosal plane, with branches terminating within the internal sphincter or extending into the intersphincteric plane
Anal transitional zone - 5-10mm, distal boundary at or just below the anal valves
- Mucosa consists of stratified columnar or stratified squamous epithelium with islands of columnar epithelium within it
Pecten zone - 10-15mm of thicker stratified squamous epithelium, devoid of hair and sebaceous and sweat glands, closely adherent to underlying tissues
Distal anal canal - hairy skin with sebaceous and sweat glands and large apocrine glands, with this area extending to the distal limit of anal margin

Physiology

Defecation
- Complex co-ordinated event - increased intra-abdominal pressure, rectal contraction, and synchronised relaxation of the anal sphincters
1. Distension of the rectum results in reflexive relaxation of the internal anal sphincter
2. Sensory epithelium in the anus sample the faecal material to distinguish between solid stool, liquid stool, and gas
3. If defecation is deemed appropriate, the external sphincter relaxes together with the puborectalis muscle, which allows straightening of the recto-anal angle, opening of the anal canal, and evacuation of faecal material.
Continence
- Requires rectal wall compliance, appropriate neurogenic control of the pelvic floor muscles, and properly functioning internal and external sphincter muscles
- Internal anal canal and external sphincters, together with haemorrhoidal cushions, provide a complete airtight and watertight seal.