These act as vertical yoke muscles but they run diagonally from their origin at the apex of the orbit to be inserted 7 mm or 8 mm behind the limbus above and below the globe. The action of these muscles depends on the initial position of the eye. For example, the primary action of the superior rectus is to elevate the abducted eye and the inferior rectus depresses the abducted eye. The secondary action of the superior rectus is to adduct and intort the adducted eye; the inferior rectus adducts and extorts the adducted eye. Intorsion and extorsion refer to rotation about an anteroposterior axis through the globe. The important thing to realise is that the action of these muscles depends on the position of the eye (Figure 14.4).
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