Anus - Anatomy & Physiology

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Structure

Anus (Sheep) - Copywright RVC 2008

• There are two anal sphincters:
  • Internal anal sphincter, formed by thickening of the circular smooth muscle of the gut and under autonomic control.
  • External anal sphincter, formed from striated skeletal muscle and under voluntary control.

Histology

Anal Glands- Copywright RVC 2008

• At the anus, the columnar intestinal epithelium is replaced by the stratified squamous keratinised epithelium of the skin.
• The anal canal joins the bowel to the exterior and is the last 2-3cm of the alimentary tract.
  • This is a short passage derived from the proctodeum (formed by invagination of the surface ectoderm).
• Sebaceous and large apocrine sweat glands both occur in this region in association with the anal sphincters.
  • Before joining the anal canal, the rectum becomes dilated to form the rectal ampulla.
• At the rectoanal junction, the lumen is constricted by longitudinal folds in the mucosa.
  • These are normally pressed together to occlude the lumen.
• 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 keratinised epithelium.
    • Surrounds the anus.
    • Excretory ducts of the anal sacs open into this region.

Species Differences

Carnivore

• The dog and cat posses two anal sacs. In the dog, these are the size of a hazlenut.
  • Located ventrolaterally between the internal and external anal sphincters.
  • Large, coiled apocrine tubules that have many tubuloalveolar glands that produce a fatty secretion in their walls.
  • The fundus of the sac secretes a potent smelling fluid that drains through a single duct to an opening near the anocutaneous juncntion.
  • The anal sacs get compressed during defecation, which causes the fluid to be expressed. The scent of the fluid is thought to act as a territorial marker.
    • Anal sacs are clinically important as they are commonly diseased in dogs - frequently, they become enlarged due to accumulated secretion.

• The dog has perianal glands that lie in the skin surrounding the anus.
  • They are modified sebaceous glands.

Pig

• Also have anal sacs.