What are the pulleys of the recti extraocular muscles?

The human orbit contains specialized musculofibroelastic tissues in and just posterior to Tenon's fascia that act as compliant pulleys and determine the pulling directions of recti EOMs. In this sense, the pulleys are the functional origins of the recti EOMs and are determinants of ocular motility. Because the muscle bellies were fixed in the orbit by MRI studies even during wide ocular excursions, it seemed apparent to Joseph Demer, who described the pulley system, that the muscles must be coupled to the orbit by a pulley system in which the pulleys are located in a position such that the insertions of the muscle are allowed to move with the eye. Magnetic resonance imaging demonstrated dense connective tissue structures within posterior Tenon's fascia near the equator of the globe adjacent to the recti EOMs. Histochemistry showed these structures to be fibroelastic EOM sleeves consisting of dense bands of collagen and elastin, suspended from the orbit. There are adjacent bands that are suspended to EOM sleeves of similar composition. Substantial smooth muscle is present in the pulley suspensions and in posterior Tenon's fascia. Tenon's fascia itself was likened to a drum head suspended at its periphery by the orbital walls.

Reference:
Demer JL, Miller JM, Poukens V, Vinters HV, Glasgow BJ. Evidence for fibromuscular pulleys of the recti extraocular muscles. Invest Ophthalmol Vis Sci. 1995 May;36(6):1125-36.