**2. The actions of the extraocular muscles**

The actions of the extraocular muscles and the relative contributions of each muscle to the various ocular positions are important to understand for planning surgery. The muscles act together in order to produce smooth eye movements.

The horizontal recti have only one primary action, while the vertical and obliques each have three actions, which vary depending on the horizontal position of the eye. The relative strengths of these actions depend upon the direction of gaze. In abduction, the vertical muscles have a vertical action only, but in adduction, they become tortors of the eye.

The superior rectus acts as an intortor in extreme adduction, but in abduction, intorsion is lost and exchanged by its elevating primary function. The inferior rectus acts as an extorter when the eye is in adduction; when the eye is abducted, it acts as a pure depressor.

The superior oblique acts as an intortor, depressor, and abductor. It is the principal intortor of the eye produced by the anterior fibers of the tendon. The posterior fibers mediate depression. In adduction, it becomes a pure depressor; in abduction, it is a pure intortor. The inferior oblique acts as extorter, elevator, and abductor. It is the principal extorter of the eye. In extreme adduction, it becomes a pure elevator; in abduction, it is a pure extortor.

These muscles act in concert with cooperation between ipsilateral and contralateral groups of muscles, abiding Sherrington's and Hering's laws. Sherrington's law of reciprocal innervation describes that contraction of a muscle is accompanied by relaxation of its ipsilateral antagonist muscle ensuring smooth movements of the eye. Hering's law of equal innervation regards binocular movements and explains that equal contractions occur in the muscles that are contralateral synergists and ensures that equal movements of the two eyes occur, if both muscles are normal.
