Fig. 31. Summary of static orbital mechanics. Family of length-tension curves as function of fixed increments of innervation (thin solid lines). Open circles mark the intersection of given eye position with length-tension curve of corresponding innervation thus establishing steady-state force needed to hold the eye at this point of gaze fixation. Dotted parabolic curve joining these points is the static locus of in situ fixation forces for any point of lateral gaze. AB, abduction; AD, adduction. Line P results when lateral rectus (LR) force and medial rectus (MR) force (i.e., the two dotted lines) are added and reflects the passive orbital torque tending to restore the eye to a central position. Dashed lines show net muscle force as eye is forcibly moved while subject attempts to hold gaze. (Redrawn after Robinson DA: In Brooks VB [ed]: Handbook of Physiology, The Nervous System. Vol II, Part 2. Baltimore: Williams & Wilkins, 1981.)