EYES OF TYPHLOPS. 57 



Cornea. — This structure is present and can be traced to the region of the 

 ciliary processes. 



Lens. — 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. — 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 

 0.0725 mm. 



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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 (0.00S 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, 0.015 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 0.016 mm. 



The outer reticular layer (4) is very thin and consists of a few fibers so arranged 

 as to leave a great number of spaces between the two nuclear layers. The distance 

 between the nuclear layers is about 0.005 mrn - 



Cones. — The sensory epithelium shows two distinct parts, an inner layer of 

 nuclei (3) and an outer row of cones (2). In the sections these two were so separated 



