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Related to tertiary action: Tertiary education
Term referring to the least effect of an extrocular muscle. The tertiary action of the inferior rectus is extorsion; of the superior rectus, intorsion; of the inferior and superior oblique, abduction. The medial and lateral recti muscles are regarded as having only a primary action, at least in the primary position.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann