The annulus of Zinn, also known as the common tendinous ring and the annular tendon, encompass the optic nerve of the eye. Located just below the superior oblique muscle, this oval band of sinewy tissue is the junction point of a group of muscles referred to as extraocular muscles -- the four muscles that allow the human eyeball to move freely in the orbit (eye socket). Three other nerves and one artery extend through the common tendinous ring. The nasocilliary nerve, which branches from the optic nerve, the abducens nerve or sixth cranial nerve and the oculomotor nerve or third cranial nerve. The one artery that passes through the ring, the opthalmic artery, supplies the eye with the necessary blood supply. The annular tendon contains two parts -- upper and lower. The upper portion, or superior tendon of Lockwood, junctions with the superior rectus. The lower portion of the ring, or tendon of Zinn, couples with the inferior rectus. Both the superior rectus and the inferior rectus are part of the medial rectus, which moves the eyes horizontally.