The medial rectus muscle is a muscle in the orbit near the eye. It is one of the extraocular muscles. It originates from the common tendinous ring, and inserts into the anteromedial surface of the eye. It is supplied by the inferior division of the oculomotor nerve (III). It rotates the eye medially (adduction).
Structure
The medial rectus muscle shares an origin with several other extrinsic eye muscles, the common tendinous ring. It inserts into the anteromedial surface of the eye. This insertion has a width of around 11 mm. It is around 5.5 mm from the corneal limbus. A branch of it enters the muscle around two fifths along its length. It rarely changes position significantly when it contracts, unlike the other extraocular muscles.
Function
The medial rectus muscle rotates the eye medially (adduction). It works using a pulley system as it curves around the anterior surface of the eye. Sometimes, botulinum toxin may be injected into the medial rectus muscle. This leaves it vulnerable to being compressed (incarcerated) during skull fractures, which can prevent movement of the eye. This usually resolves when skull fractures are fixed. The damage can be minor, such as bruising, or severe, such as cutting through the muscle partially or completely, and nerve injury.