A trilogy of the oculomotor system Part II - active, passive, and dissipative forces Part III - diagnostic tests.

In Part II, this paper addresses ocular motions, their causes (forces), and the governing laws, beginning with the fundamental question: Why do the eyes move? Ocular rotations and different types of translations (ocular, orbital, and corporeal) are reviewed. The discussion then turns to how the eyes move, where concepts such as the plane of muscular action, torque, and arc of contact provide possible explanations for the anatomical arrangement of the ocular muscles within the orbit. Sherrington's law of reciprocal innervation is used to explain the distribution of muscular active forces in a conservative mechanical system, but in combination with Hering's law, it may prevent eye rotation (e.g., isometric contractions of antagonist muscles of an asymmetrical convergence). Normally, however, the limitation of an eye rotation is determined by passive forces, evoked by muscular activity itself, particularly natural muscular elasticity. Thus, elongation of an antagonist muscle may passively restrict the active contraction of an agonist. In addition to mechanisms for initiating rotation (active forces) and stopping it (passive forces), the oculomotor system also requires a means of dissipating energy (dissipative forces) to initiate subsequent movements. Hence, it cannot function as a perfectly conservative system of forces. The paper concludes with a review of "selective" effects of muscle function (due to the sparse distribution of fibers), the role of intermuscular membranes (and pulleys), and mechanical considerations of surgical procedures, such as muscular transpositions to alter or abolish actions (e.g., bifid reinsertions). Part III will address the diagnostic complexities of the oculomotor system, general treatment principles, and ocular fixation (eye and head positions). Although the basic concept of the primary position is relativized, the absolute need for referential conditions in defining, qualifying, and measuring strabismus is emphasized. The prim-diopter is challenged due to its lack of "linearity" relative to angular units, and an alternative is proposed. Methods of examining oculomotor disturbances are outlined, including monocular rotations (ductions), and tests to differentiate between muscular deficiencies and opposing forces. Techniques for identifying the site of a rotational restriction are described, followed by approaches to measuring ocular deviations in diagnostic positions. The concepts of muscular overactions and underactions are analyzed before introducing the concept of diagnostic muscle pairs. Classical knowledge about deviations caused by deficient or restricted muscle actions reinforces the theory of distribution of rotational ocular muscles by diagnostic pairs. For vertical deviations, "underactive" muscle pairs must be separately matched (e.g., RSR with LIR, RIO with LSO). Since vertical recti exert stronger vertical actions than oblique muscles, head tilts are recommended to enhance stress on both pairs, mainly by additional stimulation of oblique muscles. Classical diagnostic directions then align with the objective horizontal plane. The article concludes with peropertative oculomotor testing and a broad protocol for evaluating oculomotor imbalance.