Anus – the opening through which the body passes solid waste (feces), below the final segment of the COLON and the terminus of the gastrointestinal system. The anal sphincter is a ring of MUSCLE that contracts to contain fecal matter and relaxes to expel it. Learning to control the contraction and relaxation of the anal sphincter begins to take place at age two or three; most children master this control by age four or five. NERVOUS SYSTEM damage, such as with PARKINSON’S DISEASE and sometimes as a consequence of aging, can cause loss of anal sphincter control with resulting FECAL INCONTINENCE.
For further discussion of the anus within the context of gastrointestinal structure and function, please see the overview section “The Gastrointestinal System.”