Our visual apparatus is a dual (paired) organ, symmetrically placed on both sides of the median plane of the body. In the skull the dual parts coalesce at two places, the chiasm and the quadrigeminal bodies which are cut by the median plane, whereas in all other places the dual parts are so arranged that each of their points on one side has a corresponding point at equal distance on the other, as if one side were a reflex image of the other, cast by a plain mirror substituting the median plane of the body.
Outside the skull, all the remaining parts, orbits and their contents, as well as the eyeballs with all their intricate structures, follow the same rule; each point has its correspondent at the same distance on the other side of the median plane of the head.
My purpose, at this place, is to consider only one
KNAPP H. THE SYMMETRY OF OUR VISUAL APPARATUS AS A DUAL ORGAN.A PLEA TO MODIFY THE CUSTOMARY NOTATION OF THE OCULAR MERIDIANS.. JAMA. 1902;XXXIX(11):612–616. doi:10.1001/jama.1902.52480370020001f
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