Anal Canal

Introduction

  1. Terminal part of the large intestine
  2. Situated below the level of Pelvic diaphragm
  3. Lies in triangle of perineum b/w left and right Ischioanal Fossa

Extent

Anorectal junction to Anus

Direction

Downwards and backwards

Relations

1. Anteriorly

In males – membranous urethra & the bulb of penis

In females – lower end of vagina

Perineal body (In both males and females)

2. Posteriorly :

Anococcygeal ligament and tip of coccyx

3. Laterally

Ischioanal fossae

4. All around :

Sphincter muscles

 

Interior of Anal Canal

1st Upper Mucous Part:

15mm long, lined by mucous membrane

Mucous membrane shows – 

a)6-10 vertical folds called Anal Columns of Morgagni

b)Lower ends of anal canal united by transverse folds called Anal Valves & above each valve is a depression in mucosa called Anal Sinus

c)Anal valves form a transverse line around the canal called Pectinate line situated opp middle of Internal Anal Sphincter, the Ectodermal and Endodermal junction (occasionally these valves show projections called anal papillae which are remnants of embryonic anal membrane)

2nd Middle or Transitional Zone or Pecten:

15 mm long lined by mucous membrane w/o anal columns.

Dense venous plexus b/w it and muscle coat gives it bluish colour.

The upper region being less mobile is called Transitional zone or Pecten.

Lower part has whitish appearance and called as White line of Hilton situated at level of interval b/w subcutaneous part of external anal sphincter and lower border of internal anal sphincter.

3rd Lower Cutaneous Part:

8mm long lined by true skin with sebaceous, sweat glands and hair.

Development

Upper 2/3rd

Primitive Anorectal Canal

Lower 1/3rd

Below Pectinate line

Proctodeum

Blood Supply & Venous Drainage

 Venous Drainage:

1)Plexuses – Internal Rectal and External Rectal Venous Plexus

2) Internal Plexus lies in Sub mucosa of Anal Canal where as External lies outside Muscular Coat or Rectum and Anal Canal.

3) Internal Plexus drains into Superior Rectal Vein and communicates freely with External Plexus thus with middle and inf rectal veins.

4) Lower part of External Plexus is drained by Inf Rectal Vein into Internal Pudendal Vein;

5) Middle part by Middle Rectal Vein into Internal Iliac Vein and

6) Upper Part by Superior Rectal Vein which continues as Inferior Mesenteric Vein

 

7)Anal Veins are arranged radially around anal margin and communicate freely with Internal Plexus and Inferior Rectal veins thus causing External Piles due to rupturing of one of veins by excessive straining during defecation.

Blood Supply:

Above Pectinate line

Superior Rectal Artery

Below Pectinate line – 

Inferior Rectal Artery 

 

Lymphatic drainage:

Above Pectinate line – 

Internal iliac nodes

Below Pectinate line – 

Medial group of Superficial Inguinal nodes

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