Internal anal sphincter

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Science and technology

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What Wikipedia says

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. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum.

Excerpt from the Wikipedia article “Internal anal sphincter”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: Henry Vandyke Carter, Public domain, via Wikimedia Commons.