It is the external opening of rectum
Two sphincters control the exit of feces from the body during an act of defecation,
which is the primary function of the anus.
These are the internal anal sphincter and the external anal sphincter,
which are circular muscles that normally maintain constriction of the orifice and which relaxes as required by normal physiological functioning.
The inner sphincter is involuntary and the outer is voluntary.
Function : Defecation