Medial rectus muscle

The medial rectus is one of the four recti muscles of the eye and is the largest among them. It lies along the medial wall of the orbit and is primarily responsible for turning the eyeball toward the midline (adduction).

Origin and Insertion

• Origin: Common tendinous ring (annulus of Zinn) at the apex of the orbit.

• Insertion: Medial surface of the sclera, anterior to the equator of the eyeball, approximately 5.5 mm posterior to the corneal limbus.

Note: Like all recti muscles, the medial rectus attaches to the anterior half of the globe, whereas the oblique muscles attach posterior to the equator.

Nerve Supply

• Oculomotor nerve (cranial nerve III) — inferior division.

Actions

• Primary action: Adduction — moves the eyeball medially toward the nose.

• The medial rectus has no significant secondary actions, as its line of pull is almost directly along the horizontal (transverse) axis of the eye.

Functional Note

The medial rectus runs nearly parallel to the visual axis when the eye is in the primary position (looking straight ahead). Unlike the superior and inferior rectus muscles, it is not angled upward or downward, and therefore its line of pull coincides closely with the horizontal plane of the orbit.

Because of this direct alignment:

• When it contracts, the eyeball rotates medially (around its vertical axis), producing pure adduction.

• It does not elevate, depress, or rotate (torsion) the globe.

In simple terms, the medial rectus pulls the eye straight toward the nose, without any upward, downward, or twisting movement.

Clinical Correlation

To test the medial rectus, the examiner asks the patient to look medially (toward the nose). This isolates the muscle’s action, as the line of pull is directly aligned with the visual axis, producing pure adduction.