Fig. 6. Each muscle can be considered to run in a straight line from its origin (O), to a point of tangency (T), where it first touches the globe, and thence in a great circle path called the arc of contact to the anatomic insertion (A). The center for rotation of the globe (C), and these points define the “muscle plane,” or TACO. When the muscle pulls, it rotates the globe in the muscle plane around an axis perpendicular to the muscle plane. The effective point of insertion is point T, which changes as the eye rotates into various directions.