Anus - Anatomy & Physiology

Overview

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: The internal anal sphincter, formed by the thickening of the circular smooth muscle of the gut and under autonomic control and the external anal sphincter, formed from striated skeletal muscle and under voluntary control.

Function

The function of the anus is 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. They are 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 glands 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.

Equine

Click here for information on the equine anus.

Porcine

Also have anal sacs.

Links

Click here for information on thepathology Of The Small and Large Intestines.

Click here for anus histology.