Anal Sphincter | Info

The external sphincter is a layer of voluntary (striated) muscle encircling the outside wall of the anal canal and anal opening. One can cause it to expand and contract at will, except during the early years of life when it is not yet fully developed. Nerves…

The internal anal sphincter , IAS , (or sphincter ani internus ) is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal ; its inferior border is in contact with, but quite separate from, the external anal sphincter .