Muscle pulleys change the effective origin of the extraocular muscles.

Schematic representation of orbital connective tissues. IR, inferior rectus; LPS, levator palpebrae superioris; LR, lateral rectus; M, medial rectus; SO, superior oblique; SR, superior rectus; Tndn, tendon. The three coronal views correspond to the levels indicated by arrows in the horizontal section. In the horizontal section, note the attachment of the globe to the orbit by the anterior part of Tenon's capsule (collagen and elastin) through which the extraocular muscles pass in sleeves, which serve as pulleys. (Courtesy of Joel M. Miller and Joseph L. Demer.)

MRI showing pulleys in action for left and right gaze.