Functional anatomy of normal human rectus muscles.

The actions of extraocular muscles depend on their positions as a function of gaze. These positions vary with muscle forces, which are normal only in alert subjects making voluntary fixations. Magnetic Resonance Imaging (MRI) was used to view normal human orbits, with voluntary gaze varied over a circular field 77 deg in dia, centered on the orbital axis. Computer-aided reconstructions, reflecting the data of four normal adult subjects, produced clear pictures of the rectus muscles and optic nerve, and yielded data on muscle paths and cross-sections. From their origins in the orbital apex to their points of tangency with the globe, rectus muscle side-slip, relative to the orbit, is approximately zero; consequently, their "muscle planes" (though not necessarily their axes of rotation) are approximately fixed in the orbit. As the rectus muscles contract they draw in towards the orbital axis, and as they relax they bow outwards; this excursion is as large as 3.7 mm. Contraction also tends to cause the planes of maximum cross-section to move posteriorly.