Which muscle has excyclotorsion as its secondary action?

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

Answer: (B)

Excyclotorsion is the outward rotation of the top of the eye around the visual axis. Each extraocular muscle has a primary action (the main movement) and secondary actions that arise from its pull around the eye’s axes. The inferior rectus mainly depresses the eye, but because of where it attaches on the globe, its pull also causes the eye to rotate outward around the optic axis. That outward rotation is excyclotorsion, making it the secondary torsional action of this muscle. Other muscles contribute torsion in different directions (for example, the superior rectus mainly intorts, and the superior oblique is a primary intorter and depressor), so the inferior rectus is the one whose secondary action includes excyclotorsion.