Medial rectus muscle

At a glance

English

Medial rectus muscle

Latin (TA2)

Musculus rectus medialis

System

Muscle

Region

Head

Side

Paired, left and right

About the medial rectus muscle

Among the extraocular muscles, the medial rectus is the main adductor of the eye, turning its front inwards towards the nose. It runs along the medial wall of the orbit, between the eyeball and the thin bone separating the orbit from the ethmoidal air cells. It is the largest and strongest of the recti, though it is shorter than the others, and it works as the partner of the lateral rectus in all side-to-side movements of the eyes.

Like the other recti it begins at the apex of the orbit, from the medial part of the common tendinous ring and the dural sheath of the optic nerve. It runs straight forwards and inserts by a broad tendon, about a centimetre wide, on the medial side of the sclera, closer to the corneal edge than any other rectus, at roughly five and a half millimetres. Because its course is parallel with the visual axis, contraction produces almost pure adduction with little lifting or rotation. Its tendon wraps around the curve of the globe, and connective-tissue pulleys in the orbit keep its path steady so that it hardly shifts as it shortens. A check ligament from its sheath to the lacrimal bone limits its pull.

It is supplied by the lower division of the oculomotor nerve, whose branch enters its outer, eyeball-facing surface about two fifths of the way along the muscle and then branches further. Blood comes from the medial, or inferior, muscular branch of the ophthalmic artery. Both medial recti contract together when the eyes converge to look at a near object.

Because it lies right against the thin medial wall, the muscle can be trapped in a fracture of that wall, restricting eye movement until the fracture is repaired, and it can be injured during endoscopic sinus surgery, from a bruise to a complete cut. A medial rectus placed too high can contribute to squint. When the lateral rectus is paralysed, the unopposed medial rectus turns the eye inwards; injecting botulinum toxin into it can correct that inward squint for a time.

Attachments, action and supply

Origin

Medial part of the common tendinous ring at the orbital apex.

Insertion

Medial side of the sclera, about 5.5 mm from the corneal limbus.

Action

Adducts the eye, turning it towards the nose; both act together in convergence.

Nerve supply

Oculomotor nerve (cranial nerve III), lower division.

Blood supply

Medial (inferior) muscular branch of the ophthalmic artery.