To make full use of the visual abilities of the eyes, almost equally as important as interpretation of the visual signals from the eyes is the cerebral control system for directing the eyes toward the object to be viewed.
Muscular Control of Eye Movements. The eye movements are controlled by three pairs of muscles, shown in Figure 51-7: (1) the medial and lateral recti, (2) the
Extraocular muscles of the eye and their Innervation superior and inferior recti, and (3) the superior and inferior obliques. The medial and lateral recti contract to move the eyes from side to side. The superior and inferior recti contract to move the eyes upward or downward. The oblique muscles function mainly to rotate the eyeballs to keep the visual fields in the upright position.
Neural Pathways for Control of Eye Movements. Figure 51-7 also shows brain stem nuclei for the third, fourth, and sixth cranial nerves and their connections with the peripheral nerves to the ocular muscles. Shown, too, are interconnections among the brain stem nuclei by way of the nerve tract called the medial longitudinal fasciculus. Each of the three sets of muscles to each eye is reciprocally innervated so that one muscle of the pair relaxes while the other contracts.
Figure 51-8 demonstrates cortical control of the oculomotor apparatus, showing spread of signals from visual areas in the occipital cortex through occipito-tectal and occipitocollicular tracts to the pretectal and superior colliculus areas of the brain stem. From both the pretectal and the superior colliculus areas, the oculomotor control signals pass to the brain stem nuclei of the oculomotor nerves. Strong signals are also transmitted from the body's equilibrium control centers in the brain stem into the oculomotor system (from the vestibular nuclei by way of the medial longitudinal fasciculus).
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