part of part of surrounds surrounds anal canal internal anal sphincter http://upload.wikimedia.org/wikipedia/commons/e/e9/Gray1079.png UMLS:C0224394 NCIT:C32833 circular muscle layer of anal canal circular layer of muscularis externa of anal canal MA:0001534 circular layer of anal muscularis externa UBERON:0001244 The smooth muscle ring, formed by an increase of the circular muscle fibers of the rectum, situated at the upper end of the anal canal, internal to the outer voluntary external anal sphincter. circular layer of muscularis propria of anal canal EMAPA:18266 Wikipedia:Internal_anal_sphincter FMA:15710 anal region anal region smooth muscle anal sphincter smooth muscle sphincter circular muscle layer of muscular coat