Excerpt from Vision

Although each eye has its own supply of muscles and nerves, yet the two eyes are not independent of each other in their movements. The nature of their connections with the nerve-centres is such that only those movements are, as a rule, possible in which both axes of vision remain in the same plane. This condition being fulfilled, the eyes may be together directed to any desired point above, below, or at either side of the observer. The axes may also be con verged, as is indeed necessary in looking at near objects, and to facilitate this convergence the internal recti muscles are inserted nearer to the cornea than the other muscles of the eye. Though in the ordinary use of the eyes there is never any occasion _to diverge the axes of vision, yet most persons are able to effect a divergence of about four degrees, as shown by their power to overcome the ten deney to double vision produced by holding a prism in front of one of the eyes. The nervous mechanism through which this remarkable cc-ordination of the muscles of the two eyes is effected, and their motions limited to those which are useful in binocular vision, is not completely understood, but it is supposed to have its seat in part in the tubercula quadrigemina, in connection with the nuclei of origin of the third, fourth, and sixth cranial nerves. Its disturbance by disease, alcoholic intoxication, etc. Causes strabismus, confusion, dizziness, and double vision.

A nerve termination sensitive to light, and so arranged that it can be turned in different directions, is sufficient to give information of the direction from which the light comes, for the contraction of the various eye-muscles indicates, through the nerves of muscular sense, the position into which the eye is nor mally brought in order to best receive the luminous rays, or, in other words, the direction of the luminous body. The eye, however, informs us not only of the direction, but of the form of the object from which the light proceeds; and to understand how this is effected it will be necessary to consider the refracting media of the eye by means of which an optical image of the luminous object is thrown upon the expanded termination of the optic nerve - viz. The retina.

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