. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Diagram to represent the supposed identical part* of the two retinae. {After Midler.) 1, 2, 3, &c., the identical parts of the two retinae ; c, c, the optic axes. retinae occupy the same degrees of latitude and longitude: 1 is identical with 1, 2 with 2, and so on ; but 1 in the one eye is not identical with 5 in the other eye. " To explain the single vision, therefore, it is necessary that not merely each root of the optic nerve, but each primitive fibre of each root should in the chiasma divide into two branch
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Diagram to represent the supposed identical part* of the two retinae. {After Midler.) 1, 2, 3, &c., the identical parts of the two retinae ; c, c, the optic axes. retinae occupy the same degrees of latitude and longitude: 1 is identical with 1, 2 with 2, and so on ; but 1 in the one eye is not identical with 5 in the other eye. " To explain the single vision, therefore, it is necessary that not merely each root of the optic nerve, but each primitive fibre of each root should in the chiasma divide into two branches for the two optic nerves, so that the identical fibres of the two nerves might com- municate with the brain at one point only, viz. by one radical fibre, as in tiie annexed wood- cut (fg. 422). But such a division of the fibres in the chiasma does not exist: Treviranus and Volkmann were unable to detect any division of fibres in the chiasma, and I also was unsuc- cessful in my search for such dividing fibres. (Fig. 422. Fig. Diagram to represent an ideal division of the fibres in the chiasma suitable to this theory. (After Miiller.) a, a, optic nerves ; h, b, tractus optici ; c, c, sup- posed division of each radical fibre in the chiasma into two branches, one for each optic nerve. 4. If single vision in man be explained on the assumption that certain parts of the two retinae are reciprocally identical, and that such identity depends upon a partial decussation in the chiasma, single vision in animals should of course admit of explanation upon the same principles; and if this be granted, the relative directions of the optic axes in the vertebrate classes ought to afford a good criterion of the extent to which the retinae are reciprocally identical; for when the optic axes have a strictly lateral direction (as in many osseous fish), the same object can never be depicted on both retinae simultaneously, and consequently it may be inferred that, in such cases, no parts of
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