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The Sense of Sight / All About the Eyes
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submitted by Dr. Gary Farr - Contact the author here.
Last Updated June, 14, 2002
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The binocular movements (the movements of the two eyes) fall into two classes, the conjugate movements, when both eyes move in the same direction, as in a change in the direction of gaze, and disjunctive movements, when the eyes move in opposite directions. Thus, during convergence onto a near object both eyes move toward the nose; the movement is horizontal, but disjunctive, by contrast with the conjugate movement when both eyes move, say, to the right. The disjunctive movement of convergence can be carried out voluntarily, but the act is usually brought about reflexly in response to the changed optical situation—i.e., the nearness of the object of gaze. A seesaw movement of the eyes, whereby one eye looks upward and the other downward, is possible, but not voluntarily; to achieve this a prism is placed in front on one eye so that the object seen through it appears displaced upward or downward; the other eye sees the object where it is. The result of such an arrangement is that, unless the eye with the prism in front makes an upward or downward movement, independent of the other, the images will not fall on corresponding parts of the retinas in the two eyes. Such a noncorrespondence of the retinal images causes double vision; to avoid this, there is an adjustment in the alignment of the eyes so that a seesaw movement is actually executed. In a similar way, the eyes may be made to undergo torsion, or rolling. A conjugate torsion, in which both eyes rotate about their anteroposterior (fore-and-aft) axes in the same sense, occurs naturally; for example, when the head tips toward one shoulder the eyes tend to roll in the opposite direction, with the result that the image of the visual field on the retina tends to remain vertical in spite of the rotation of the head.
The nerves controlling the actions of the muscles are the third, fourth, and sixth cranial nerves, with their bodies (nuclei) in the brainstem; the third, or oculomotor nerve, controls the superior and inferior recti, the medial rectus, and inferior oblique; the fourth cranial nerve, the trochlear nerve, controls the superior oblique; and the sixth, the abducens nerve, controls the lateral rectus. The nuclei of these nerves are closely associated; especially, there are connections between the nuclei of the sixth cranial nerve, controlling the lateral rectus, and the nucleus of the third, controlling the medial rectus; it is through this close relationship that the linking of the lateral rectus of one eye and the medial rectus of the other, indicated above, is achieved. Another type of linking is concerned with reciprocal inhibition; that is, when there are two antagonistic muscles, such as the medial and the lateral rectus, contraction of one is accompanied by a simultaneous inhibition of the other. Muscles show a continuous slight activity even when at rest; this keeps them taut; this action, called tonic activity, is brought about by discharges in the motor nerve to the muscle. Hence, when the agonist muscle contracts its antagonist must be inhibited. 
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Additional information regarding conditions of the eyes can be found here.
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