The iris sphincter is best described as the...

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Multiple Choice

The iris sphincter is best described as the...

The iris sphincter is the circular muscle around the pupil that constricts it. When this sphincter contracts, the pupil becomes smaller (miosis), which is controlled mainly by parasympathetic signals. The opposing pupil-dilating action comes from the dilator pupillae, which has radially arranged fibers and widens the pupil under sympathetic influence. The other options either describe the muscle that opens the pupil (the dilator, not the sphincter), misstate the muscle’s shape, or refer to tissue not involved in controlling pupil size (the corneal stroma). So the best description is a round circular muscle that closes the pupil.

The iris sphincter is the circular muscle around the pupil that constricts it. When this sphincter contracts, the pupil becomes smaller (miosis), which is controlled mainly by parasympathetic signals. The opposing pupil-dilating action comes from the dilator pupillae, which has radially arranged fibers and widens the pupil under sympathetic influence. The other options either describe the muscle that opens the pupil (the dilator, not the sphincter), misstate the muscle’s shape, or refer to tissue not involved in controlling pupil size (the corneal stroma). So the best description is a round circular muscle that closes the pupil.

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