The medial rectus muscle is the largest of the eye’s extraocular movement muscles, six individual muscles that surround the eye and help control the eye's movement. The other five extraocular muscles are the lateral rectus, superior oblique, superior rectus, inferior rectus, and the inferior oblique.
Specifically, the medial rectus muscle works to keep the pupil closer to the midline of the body. It helps move the eye up and down and from side to side. It also works with the two oblique muscles, whose function is to move the eye in and out.
When the medial rectus muscle is not working correctly and eye movement is affected it is called strabismus, which is a condition where the two eyes are misaligned and, therefore, point in different directions. Strabismus can be corrected by ophthalmologists, who are doctors that specialize in eye conditions.