Which muscle classification is described as a circular shape and function as a sphincter?

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Multiple Choice

Which muscle classification is described as a circular shape and function as a sphincter?

Explanation:
Circular muscles are arranged in a ring around an opening, and their contractile action constricts that opening to regulate passage, functioning as a sphincter. This ring shape lets them tighten and close openings like the mouth, eyelids, or various internal passages (examples include the orbicularis oris around the mouth and the iris’ smooth muscle controlling pupil size). The other forms don’t form a surrounding ring: pennate muscles have fibers set at angles to a tendon to maximize force; fusiform muscles are spindle-shaped with parallel fibers optimized for shortening; quadrate muscles are four-sided and flat. So the ring-shaped arrangement that constricts an opening best fits a circular muscle acting as a sphincter.

Circular muscles are arranged in a ring around an opening, and their contractile action constricts that opening to regulate passage, functioning as a sphincter. This ring shape lets them tighten and close openings like the mouth, eyelids, or various internal passages (examples include the orbicularis oris around the mouth and the iris’ smooth muscle controlling pupil size). The other forms don’t form a surrounding ring: pennate muscles have fibers set at angles to a tendon to maximize force; fusiform muscles are spindle-shaped with parallel fibers optimized for shortening; quadrate muscles are four-sided and flat. So the ring-shaped arrangement that constricts an opening best fits a circular muscle acting as a sphincter.

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