The morphological principles of anorectal continence.
Anal continence is depending on the rectum, the anal skin, the external anal sphincter and the levator ani as well as the corpus spongiosum of the rectum and the internal anal sphincter. Continuous contraction of a sphincter without fatigue can only be accomplished by smooth muscles. The internal anal sphincter in man does not contain any ganglion cells. It differs from the gastro-intestinal tract by not containing the so-called p-fibres. This electron microscopic difference between the internal anal sphincter and the rest  of the gastrointestinal tract can also be demonstrated pharmacologically. Cholinergic and adrenergic fibres will both produce contraction of the internal sphincter. Comparative anatomical studies also show the uniqueness of this sphincter. For instance in the kangaroo the internal anal sphincter is the main sphincter of the anus. The sphincter system, like all other systems, is unable to regenerate and is therefore more than adequately provided for. For instance continence will be maintained even when the greatest part of the ring musculature has to be divided in patients with high ano-rectal fistulae.
D000293:Adolescent; D001003:Anal Canal / Q000033:anatomy & histology* / Q000502:physiology; D005260:Female; D006801:Humans; D009119:Muscle Contraction; D009130:Muscle, Smooth / Q000502:physiology / Q000648:ultrastructure; D012007:Rectum / Q000033:anatomy & histology* / Q000502:physiology