. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Heteropygii. EYES OF TYPHLOPS. 57 Cornea. â This structure is present and can be traced to the region of the ciliary processes. Lens. â A large lens is present, its depth being equal to about two-fifths of the eye depth. From the sections little could be determined about its structure. A well-developed capsule surrounds it (plate 4, fig. c). Retina. â The same layers are present that are found in snakes in general, but the comparative thickness of the various layers is different. In the garter snakes, for instance,
. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Heteropygii. EYES OF TYPHLOPS. 57 Cornea. â This structure is present and can be traced to the region of the ciliary processes. Lens. â A large lens is present, its depth being equal to about two-fifths of the eye depth. From the sections little could be determined about its structure. A well-developed capsule surrounds it (plate 4, fig. c). Retina. â The same layers are present that are found in snakes in general, but the comparative thickness of the various layers is different. In the garter snakes, for instance, the retina is of a uniformly even thickness even to the ciliary process, a single layer of cells continues on over the surface of the processes and iris, but in Typhlops lumbricalis the retina at the back of the eye is very thick and gradually becomes thinner till it ends a short distance from the ciliary processes (plate 4, fig. c). At this point the arrangement could not be definitely determined in the sections. At the back the retina, exclusive of the pigment layer, measures mm. tiWhl :, e^M^ill" 3 i $£334 f, &â ; a. Fig. 21. (a) Section of the Retina of an Adult Specimen, 21 cm. long. (6) Section of the Retina of a Specimen, 10 cm. long. Ends of fibers were seen projecting inward from the ganglion-cell layer, but no definite fiber layer could be distinguished (10 in fig. 21 b). The ganglion-cell layer (9 in the figures) consists of a single row of large nucleated cells, somewhat irregularly arranged ( mm.). The inner reticular layer (8) consists of a mass of fibers interwoven in a close network. This layer measures, at the back of the eye, mm. The inner nuclear layer (6) consists of at least 3 layers of cells, loosely arranged. The course of some of the fibers can be followed among these cells. This layer measures mm. The outer reticular layer (4) is very thin and consists of a few fibers so arranged as to leave a great number of
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