Inferior rectus muscle

Last revised by Craig Hacking on 27 Dec 2021

The inferior rectus muscle is one of the six extraocular muscles that control eye movements.

Inferior rectus, along with the other rectus muscles, arises from the annulus of Zinn, the common tendinous ring at the apex of the orbit that surrounds the optic canal 1.

Inferior rectus runs anteriorly on the inferior surface of the eye and inserts into the inferior surface of the sclera just posterior to the junction of cornea and sclera 2.

Inferior rectus is crossed by the inferior oblique muscle, which runs inferior to it as it crosses the floor of the orbit.

Branches of the ophthalmic artery, itself a branch of the internal carotid artery.

Innervated by the oculomotor nerve, which also supplies medial rectus, superior rectus, and inferior oblique muscles.

The primary action of the inferior rectus is to depress the eye (see figure 1) 1. However, because the apex of the orbit is placed medially in the skull, the orbital axis that the inferior rectus runs in does not correspond with the optical axis of the eye in its neutral position. This means that the inferior rectus has secondary actions of adduction and external rotation (see figures 2 and 3).

If the eye is abducted by the lateral rectus such that the optical axis lines up with the orbital axis, the inferior rectus produces ocular depression only, and is solely responsible for this movement. Thus, when the physician testing eye movements first asks the patient to follow their finger laterally then inferiorly in the familiar H-shape, the inferior rectus muscle (and the oculomotor nerve that supplies it) are being directly tested.

If the eye is adducted by the medial rectus, the orbital axis runs almost perpendicular to the optical axis, so the inferior rectus no longer produces effective ocular depression, and instead produces external rotation and adduction.

Rectus comes from the Latin rectos, meaning straight 1.

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