What is the tertiary action of the inferior rectus?

• A. Abduction
• B. Elevation
• C. Adduction
• D. Depression

Answer: (C)

When a single extraocular muscle is considered, it can produce three components of movement: the primary action, a secondary action, and a tertiary action. For the inferior rectus, the primary action is to depress the eye. It also contributes to adduction as a secondary action. The tertiary action, arising from how the muscle pulls as the eye moves and its rotational effect on the globe, is extorsion (outward rotation of the top of the eye).

So the tertiary action of the inferior rectus is extorsion, not adduction. Adduction is the secondary action. This reflects how each muscle has a main vertical component, a horizontal component, and a torsional component depending on gaze position. If a source lists adduction as the tertiary action, that would reflect a different convention, but the standard three-action scheme assigns extorsion as the tertiary action.