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There are two sphincters, the internal which is a thickening of the
continuration of the circular muscle of the rectum. And the external which
forms a complex with the pubo-rectalis and levator ani. Red levator ani,
green, circular muscle of rectum and internal sphincter, cyan external
sphincter, dark blue longitudinal muscle of rectum.
The anal canal has two main sphincters, these are termed the interal and
external anal sphincters.
Adrian P. Ireland