Anus - Anatomy & Physiology

Introduction

At the anus, the columnar intestinal epithelium is replaced by the stratified squamous epithelium of the skin.
As the muscosa changes from columnar to cutaneous, three zones are created:
- Columnar
  - Has many longitudinal folds.
  - Divided from the rectum by the anorectal line.
    - This is a line where the mucosa is stratified squamous epithelium containing lots of lymphoid tissue.
- Intermediate
  - Divided from the cutaneous zone by the anocutaneous line.
- Cutaneous
  - Skin.
  - Stratified squamous epithelium.
  - Surrounds the anus.
  - Excretory ducts of the anal sacs open into this region.
    - Anal sacs are large, coiled apocrine tubules.
    - Anal sacs have many glands in their walls.
    - These glands are tubuloalveolar and produce a fatty secretion.
    - Anal sacs are clinically important as they are commonly diseased in dogs - frequently, they become enlarged due to accumulated secretion.