medial rectus
Also called MR, medial rectus muscle, internal rectus
The horizontal extraocular muscle on the nasal side of the globe. It arises from the annulus of Zinn, inserts 5.5 mm behind the nasal edge of the cornea, the closest insertion of the four rectus muscles, and its only action from primary position is adduction, turning the eye inward1. The two medial recti working together are the muscles of convergence2.
Its antagonist is the lateral rectus of the same eye. The inferior division of the oculomotor nerve, the third cranial nerve, supplies it2,3. An isolated loss of adduction with convergence still intact is not a medial rectus palsy but an internuclear ophthalmoplegia, a lesion of the brainstem pathway that drives the muscle.
More detail
In a version its yoke is the lateral rectus of the other eye; in convergence its partner is the other medial rectus. Weakness produces an exotropia that is largest in gaze toward the nose. A long-standing sixth nerve palsy can tighten the medial rectus into a contracture, which then restricts abduction and gives positive forced ductions4. The nerve enters the muscle 15 mm from the origin2.
Sources (4)
- Kanski, Clinical Ophthalmology (2024), ch. 18
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 3 and ch. 4
- Adler's Physiology of the Eye, ch. 7, Table 7.1
- Yanoff and Duker, Ophthalmology (2023), ch. 9
Reviewed October 2026