Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The medial rectus muscle is a muscle in the orbit. As with most of the muscles of the orbit, it is innervated by the inferior division of the oculomotor nerve (Cranial Nerve III). This muscle shares an origin with several other extrinsic eye muscles, the anulus tendineus, or common tendon. It is the largest of the extraocular muscles and its only action is adduction of the eyeball. Its function is to bring the pupil closer to the midline of the body.