PURPOSE. Connective tissue structures constrain paths of the rectus extraocular muscles (EOMs), acting as pulleys and serving as functional EOM origins. This study was conducted to investigate the relationship of orbital and global EOM layers to pulleys and kinematic implications of this anatomy. METHODS. High-resolution magnetic resonance imaging (MRI) was used to define the anterior paths of rectus EOMs, as influenced by gaze direction in living subjects. Pulley tissues were examined at cadaveric dissections and surgical exposures. Human and monkey orbits were step and serially sectioned for histologic staining to distinguish EOM fiber layers in relationship to pulleys. RESULTS. MRI consistently demonstrated gaze-related shifts in the anteroposterior locations of human EOM path inflections, as well as shifts in components of the pulleys themselves. Histologic studies of human and monkey orbits confirmed gross examinations and surgical exposures to indicate that the orbital layer of each rectus EOM inserts on its corresponding pulley, rather than on the globe. Only the global layer of the EOM inserts on the sclera. This dual insertion was visualized in vivo by MRI in human horizontal rectus EOMs. CONCLUSIONS. The authors propose the active-pulley hypothesis: By dual insertions the global layer of each rectus EOM rotates the globe while the orbital layer inserts on its pulley to position it linearly and thus influence the EOM's rotational axis. Pulley locations may also be altered in convergence. This overall arrangement is parsimoniously suited to account for numerous aspects of ocular dynamics and kinematics, including Listing's law. (Invest Ophthalmol Vis Sci. 2000;41: 1280 -1290 I nitial attempts to mathematically model binocular alignment showed the importance to extraocular muscle (EOM) action of EOM paths and the pivotal mechanical role of orbital connective tissues. The need for EOM path data motivated early radiographic studies in monkeys 1 and humans, 2 suggesting that paths of rectus EOMs are stabilized relative to the orbit. A decade ago, Miller 3 used relatively low-resolution MRI with three-dimensional (3-D) reconstruction to demonstrate stability of rectus EOM belly paths throughout the oculomotor range
