Which muscle has abduction as its tertiary action?

• A. Inferior Oblique
• B. Superior Oblique
• C. Lateral Rectus
• D. Medial Rectus

Answer: (A)

Abduction as a tertiary action reflects how some eye muscles contribute to outward movement only after their primary and secondary roles are accounted for. The lateral rectus is the main abductor, so its primary action is outward movement, not a tertiary one. The medial rectus does the opposite, pulling the eye inward. Among the oblique muscles, abduction is not their primary function; they have three actions each, with abduction listed as a later (tertiary) action. Specifically, the inferior oblique elevates the eye and extorts it, and its third action is abduction, moving the eye outward. That combination makes it the muscle whose abduction role fits the description of a tertiary action.