56 



BLIND VERTEBRATES AND THELR EYES. 



po.s. 



pas. 



as. 



(a) Dorsal View of Head of Typhlops, 21 cm. long 

 (6) Lateral View of Head of same Specimen. 



THE EYES OF TYPHLOPS LUM BRICA LIS. 



The eye shows through the large ocular scale, which entirely covers it. It appears 

 as a black spot surrounded by an unpigmented circle. The preocular, also a large 

 scale, overlaps the ocular and reaches just to the edge of the eye (figs. 20 a, b). 



Compared with one of the garter 

 snakes and in proportion to the size 

 of the head, the eye of TypJilops lum- 

 bricalis is situated farther from the 

 surface and occupies far less space, 

 while Harder's gland, associated with 

 the eye in both, is relatively much 

 larger in Typhlops. In a specimen of 

 Typhlops luiubricalis 21 cm. in length, 

 the eye measured 0.306 mm. in width, 

 and 0.387 mm. in depth. The greatest width of the gland of the same was 0.71 1 

 mm. and the length was 1.067 mm. The gland completely surrounds the eye up 

 to the edges of the conjunctival sac (plate 4, figs. A, b). In proportion to the 

 size of the eyes, the gland of the garter snake is much smaller than that of 

 Typhlops litmbricalis, but compared with Rhineura floridana the gland of Typhlops 

 liinibriculis is but little more than half as large. 



The eye is covered by layers of epidermis and dermis that differ from these 

 same layers on neighboring parts by being thinner, more compact, and free from 

 pigment and glands. The ocular scale, however, which covers the eye region, 

 does not differ in thickness from the other scales of the head (plate 4, fig. a). 



A conjunctival sac is present with a diameter at least as great as the greatest 

 width of the eye bulb. The conjunctiva, which forms this sac, is very thin over 

 the cornea and next to the brille, where it measures 0.003 mni - At the edge of the 

 sac it is differentiated into glands, the fornix conjunctiva, and measures 0.016 mm. 

 (plate 4, figs, b and c, F. cj.). 



In horizontal section, the eye axis is seen to be turned forward about 30 away 

 from a line at right angle to the horizontal axis of the body. 



Eye muscles are present, but from the sections used, the exact number could 

 not be determined. 



Choroid and Sclera. - - The dense pigmentation makes it impossible to dis- 

 tinguish between the different coats at every point. Beyond the retina with its 

 pigment layer is an open vascular space, and this is followed by another dark layer, 

 the two together representing the choroid. The choroidal pigmentary layer seems 

 to consist of long fibers circularly arranged. The sclera can be followed by start- 

 ing with the outer covering of the optic nerve and tracing its continuation 

 about the eve. 



Iris and Ciliary Processes. — Here again the pigmentation makes it difficult 

 to determine the structure. Both iris and ciliary processes are present, for the 

 black layer extends over the anterior surface of the lens, leaving a pupil equal in 

 diameter to about one-fourth of the circumference of the lens. At points near 

 the equator of the lens this dark layer is enlarged into the ciliary processes and in 

 connection with the capsule helps to hold the lens in place. 



