Internal Sphincter Muscle
Part of the muscle around the anal canal that is involved with continence (i.e. the ability to hold on to stools until a socially acceptable time). It is sometimes damaged during childbirth and this damage is one of the causes of incontinence. The internal anal sphincter is important in keeping the anal canal closed on a day to day, minute by minute basis. It is made of special muscle fibres adapted for this job that do not fatigue or get tired.