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INTRODUCTION

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In examining and treating motor anomalies (of the eye), one never loses an uneasy feeling of incompetence until he has become thoroughly familiar with the physiologic fundamentals from which the signs and symptoms of those anomalies are to be derived. Therefore, a discussion of motor anomalies of the eyes should begin with a synopsis of the physiology of the sensorial and motor apparatus of the eyes.

—Alfred Bielschowsky (1871–1940)

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I. OCULAR ALIGNMENT AND DIPLOPIA

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A. The goals of the ocular motor system

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  1. The ocular motor system finds, fixates, focuses/aligns on, and follows visual targets. In a word, the system foveates. To foveate means to align each eye so as to cause the central light ray to fall on the fovea and the entire retinal image to fall on corresponding retinal points of both eyes (Figs. 4-1 and 4-3).

  2. The eyes must continually foveate whether the target remains fixed or moves or whether the eyes remain fixed or move in any direction: horizontal, vertical, rotatory, and vergences (convergence and divergence). Foveation promotes visual acuity and a single (fused) mental image and secures the advantages of binocular stereoscopic vision for survival.

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FIGURE 4-1

Visual axes with the eyes in the primary position, fixating on infinity distance. F = fovea.

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FIGURE 4-2

Blank for drawing the visual axes when the eyes fixate on a near point.

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FIGURE 4-3

Binocular retinal image formed when the visual axes of the two eyes align properly on the center of the target. Rays along the visual axes strike the foveae. The remainder of the image falls on corresponding (but not precisely identical) temporal or nasal parts of the retina.

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B. Ocular alignment, the visual axes, and diplopia

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  1. To examine ocular alignment, start with the patient (Pt) looking straight ahead, the so-called primary position of the eyes. Theoretically, the point of fixation in the primary position is at infinity (Fig. 4-1).

  2. A line drawn from the fovea centralis of one eye to the center of its visual field defines the visual axis. This line runs through the center of the media of the eye, striking the fovea centralis without undergoing any refraction. It is the "line of sight" of that eye. In Fig. 4-1, line F-∞ defines the __________________________ of the eye.

    visual axis (or line of sight)

  3. With the eyes in the primary position, fixating on infinity, the visual axes are □ convergent/□ essentially parallel/□ divergent.

    ☑ essentially parallel (Fig. 4-1)

  4. In Fig. 4-2, draw the ...

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