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INTRODUCTION

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Seasons return; but not to me returns Day, or the sweet approach of even or morn, Or sight of vernal bloom or Summer’s rose, Or flocks, or herds, or human face divine.

—John Milton (On His Own Blindness, at Age 43)

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I. ANATOMY OF THE EYEBALL

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Learn to draw Fig. 3-1 sight unseen.

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FIGURE 3-1.

Horizontal section of the right eye, seen from above.

Graphic Jump Location
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II. DUAL ORGANIZATION IN THE OPTIC SYSTEM

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A. Two cranial nerves, II and V, convey afferents from the eye to the brain

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  1. The optic nerve, cranial nerve (CrN) II, conveys the afferent axons for two functions, the special sense of vision and pupilloconstriction.

  2. The trigeminal nerve, CrN V, conveys the afferents for general sensation:

    1. Ocular pain.

    2. Tearing reflex.

    3. Corneal reflex.

    4. Proprioception from the extraocular muscles.

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B. Two motor systems, peripheral and central, innervate the intra- and extraocular muscles

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  1. Peripheral ocular motor nerves consist of CrNs III, IV, and VI and the carotid sympathetic nerve.

  2. Central ocular motor systems control the peripheral movements. The central system find, fixate, focus/align on, and follow visual targets. CrNs III, IV, and VI innervate the extraocular muscles for these actions.

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C. Two images, a real retinal image and a mental or visual image, made by the mind

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  1. Start with an arrow as the visual target (Fig. 3-2).

    1. Each retina receives an inverted real or actual image, due to the physical optics of the eye.

    2. Neurophysiologic processing then converts this real retinal image into an abstraction called a visual image.

  2. Projection of the visual image by the mind

    1. As Fig. 3-2 shows, the light rays that form the nasal half of the retinal image come from the ❒ temporal/❒ nasal half of the object viewed.

    2. By a process of learning, we associate the point of retinal stimulation with the reverse half of space. Hence, if light rays fall on the temporal half of the retina, the mind perceives the object as located in the ❒ temporal/❒ nasal half of space.

      Image not available. nasal half of space

    3. If the image of an object falls on the nasal side of the retina, we would reach for the object in the temporal half of space. Similarly, if the image falls on the upper half of the retina, we would reach for the object in the __________________________ half of space.

      lower

    4. Thus, the law of projection of the visual image states that the mind projects the visual image derived from one half of the retina to the __________________________ half of space.

      opposite or reverse

    5. This particular law exemplifies a general law of sensation: The mind projects afferent impulses to their usual site of origin in all sensory ...

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