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Related to primary action: primary effect
Term referring to the greatest effect of an extraocular muscle in one plane. The other actions are called the subsidiary or secondary and tertiary actions. The primary action of the inferior rectus is depression; of the superior rectus, elevation; of the inferior oblique, extorsion; and of the superior oblique, intorsion. The medial and lateral recti muscles exert their primary action in the primary position, that is, pure adduction for the medial rectus and pure abduction for the lateral rectus. See forced duction test; red glass test.