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The anal canal contains two muscular sphincters, the internal anal sphincter and the external anal sphincter. Identify the external anal sphincter muscle, which is
composed of striated muscle. It is composed of three parts (Figure 43.6). Its voluntary contraction maintains the rectal continence. The involuntary internal anal sphincter is
composed of thick circular layers of smooth muscle which is continuous with the smooth muscle of the anal canal.

Links and References:
Grant's: 3.42
Netter (1ed.): 369 (2ed.): 365
Rohen/Yokochi: 335