Extraocular Muscles

Figure.9-2 represents a schematic diagram of the orbits viewed from above, with the eyes looking to the right. The superior rectus and superior oblique muscles are shown with their insertions on the top of the globes--the superior rectus muscle elevates the globe and the superior oblique muscle depresses the globe. The inferior rectus and oblique muscles insert at homologous sites on the bottom of the globe, and the horizontal vectors of pull are the same, except the inferior rectus muscle depresses the globe while the inferior oblique muscle elevates it. The adducted position of the left eye places the superior oblique in a position to depress the eye as a primary and direct action (see also Figure 9-2 B and Tabie 9:2 ). The abducted position of the right eye puts the right superior rectus in position to elevate the right eye as a primary and direct action. The direction of pull of the left superior rectus muscle on the adducted left eye puts it in position to intort the eye as a primary and direct action (incyclodeviation or intorsion, rotation of the 12 o'clock meridian toward the nose or midline about the Z axis [see Fig 9-1 ]). Similarly, the direction of pull of the right superior oblique muscle on the abducted right eye puts it in position to intort the right eye as its primary and direct action. More generally, the oblique muscles act as the pure depressor and elevator of the eye when it is in adduction, and the rectus muscles act as the pure depressor and elevator of the eye when it is in abduction. Conversely, the primary torsional action is carried out by the rectus muscles when the eye is in adduction and by the oblique muscles when the eye is in abduction. When the eyes are in primary position, the oblique and the rectus muscles both perform a mixture of vertical and torsional action. The action of the medial and lateral rectus muscles is simple relative to the vertical muscles. The medial rectus muscle adducts and the lateral rectus muscle abducts the eye.

The importance of these relationships lies in the fact that the binocular relative deviation is largest when the patient attempts to look into the direction of pull of the defective muscle. By determining the angles of deviation between the visual axes in all the cardinal positions of gaze, the weak muscle can be identified as the one that serves the cardinal position with the largest deviation.

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