Anus - Anatomy & Physiology

Introduction

The anus is the terminal portion of the alimentary tract which communicates with the external environment. Two sphincters control it's aperture. It allows faeces and gas to leave the body. Defeacation is the process where faeces are expelled from the rectum through the anus.

Structure

Anus (Sheep) - © 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.

Function

• To transmit faeces from the rectum to the external environment.
• Anal sphincters give control over the regularity of defeaction, and this may be consciously controlled in some species.
• The anal canal lubricates the passage of faeces.

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.
  • They are large, coiled apocrine tubules that have many tubuloalveolar glands in their walls.
    • These tubuloalveolar glandsw produce a fatty secretion.
  • 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.

Porcine

• Also have anal sacs.

Links

• Pathology Of The Small and Large Intestines