The iris sphincter is the round muscle that acts like a purse-string to close the pupil. Which option best describes this function?

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Multiple Choice

The iris sphincter is the round muscle that acts like a purse-string to close the pupil. Which option best describes this function?

Explanation:
The iris sphincter is a circular smooth muscle around the pupil that contracts to constrict the pupil, acting like a purse-string to make the opening smaller. This description—“a round muscle that closes the pupil”—best captures its function. The iris dilator does the opposite by widening the pupil, and describing it as a tendon or focusing only on location around the cornea doesn’t explain the constriction action.

The iris sphincter is a circular smooth muscle around the pupil that contracts to constrict the pupil, acting like a purse-string to make the opening smaller. This description—“a round muscle that closes the pupil”—best captures its function. The iris dilator does the opposite by widening the pupil, and describing it as a tendon or focusing only on location around the cornea doesn’t explain the constriction action.

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