QS Study

Gross anatomy of the anal canal

It is the terminal part of the large intestine, situated below the levels of the pelvic diaphragm. It is the last spot where stools pass through before finally exiting the body. Muscles that surround the anal canal relax to allow waste to leave your body. It serves to lubricate and transmit fecal matter as it passes from the rectum to outside the body. It helps regulate defecation and maintain continence.

  • Length – 4 cm long. It has a narrower diameter than that of the rectum to which it is joined.
  • Location – Lies in the perineum (anal triangle) in between right and left ischiorectal fossa. It is situated in the anal triangle of the perineum, between the left and right ischioanal fossa.
  • Extension – From the anorectal junction to the anus.
  • Direction – Directed downwards and backward.

Relations

Anteriorly:

  • Perineal body.
  • Bulb of the penis and spongy urethra in male and lower part of the vagina in the female.

Posteriorly:

  • Anococcygeal raphe.
  • Tip of the coccyx.
  • Fibrofatty tissue between the perineal skin and the anococcygeal raphe.

Fig: In males and females

In front:

  1. Perineal body.
  2. In male – Bulb of the penis and spongy urethra.

In female – Lower part of the posterior wall of the vagina.

Behind: Ano-coccygeal raphe, and fibro-fatty tissue between the perianal skin and the raphe.

On each side: Ischio-rectal fossa and its contents.

Interior

The interior of the anal canal is divided into three parts:

(A) Upper part:

(1) About 15 mm long,

(2) Lined by a mucous membrane (stratified columnar),

(3) Anal columns – Vertical folds (6 to 10) seen in the mucous membrane.

(4) Anal valves – Short transverse folds of mucous membrane uniting the anal columns.

(5) Anal sinus – Depression above each valve.

(6) Pectinate line – Anal valves together form a transverse line that runs all around the canal.

(B) Middle part:

(1) 15 mm long,

(2) Lined by a mucous membrane (stratified squamous),

(3) Anal columns do not present here.

(4) This region is referred to as the pectin or transitional zone.

(5) White line (of Hilton) – The lower limit of the pecten having a whitish appearance.

(C) Lower part:

  • About 10 mm,
  • Lined by true skin containing sweat and sebaceous glands.

Anal Sphincters – The upper and lower halves of the anal canal have different embryological origins, therefore they have a different blood supply, lymph drainage, and nerve supply.

(i) Internal:

a) Involuntary in nature,

b) Formed by thickened circular muscle coat.

(ii) External:

(a) Under voluntary control,

(b) Made by striated muscle.

(c) Supplied by inferior rectal nerve and the perineal branch of S4 nerve.