EYES OF THE TEXAN CAVE SALAMANDER. 



35 



the iris and extends into the depth of the vitreous cavity. Apparently on the 

 external half of the iris the pigmented layer has become rolled in and folded upon 

 itself in the interior of the eye, giving rise to a pigment mass over ioo /j. thick. No 

 such mass is present in the left eye. The pigment on the inner or upper half of 

 the iris is as in the younger stages. The choroidal pigment entering the eye is in 

 solid, vermiform strands (fig. 7 b). 



THE RETINA. 



The retina of Typhlomolgc is much simpler than that of Proteus. In the latter 

 all the layers typical of the perfect retina are still distinguishable. In the former 

 the outer reticular layer has entirely disappeared, and the layers between the rods 

 and cones and the inner reticular layer form a mass of cells that are homogeneous 

 as far as ordinary histological methods permit one to determine. There are no- 

 where the slightest evidences of any rods or cones, either in the largest or smallest 

 individual. The nuclei of the outer nuclear, the horizontal, and inner nuclear 

 layers are alike. Mullerian fiber-nuclei have not been dislinguished as such. 

 This layer consists of about five series of nuclei and measures 44 ju. in thickness in 

 the smallest (30 mm.), and 48^ in the largest (90 mm.), specimen; it is between 

 32 and 48 /x in the specimen 70 mm. long. 



The inner reticular layer is thin, but well defined. It is 6 /x. thick in the smallest 

 specimen and 16 \l in the specimen 70 mm. long. In section the ganglionic layer forms 

 a U-shaped mass of cells. In the larger specimens it is about 60 /x thick and made 

 up of from five to seven series of cells. The vitreous cavity is a widely flaring, 

 trumpet-shaped structure, with its pointed end reaching to near the center of 

 the eye (fig. 7 a). In the older specimens it is filled by fibers and cellular tissue, 

 apparently continuous with the choroid ingrowth from the pupil (fig. 7 b). 



The optic nerve is 1 7 ju. in diameter in 

 the 30 mm. specimen. In the largest spec- 

 imen it is 24 jx thick without its sheaths. 

 At its passage through the pigmented 

 layer of the retina it is contracted to a 

 width of but 14 /a. Within this layer it 

 expands to 28 p. After passing directly 

 through the ganglionic layer it is distrib- 

 uted to the cells of this layer, some of the 

 fibers being bent at an acute angle to reach 

 the cells near the entrance of the nerve 

 into this layer. A large number of iso- 

 lated pigment granules are found associated 

 with the nuclei of the optic nerve within 

 the eye from its entrance to the gan- 

 glionic layer. There is no sheath of pig- 

 ment such as that found in Typhlogobius. Pigment cells are also occasionally 

 present in the very center of the eye (fig. 7 a 2), and are presumably associated 

 with the optic nerve. The sheath of the optic nerve consists of a direct continua- 

 tion of the choroid layer, which is for a shorter distance pigmented, and of a 

 continuation of the sclera (fig. 8). 



Blood-vessels do not enter the eye with the nerve, and none were with cer- 

 tainty detected except in the largest individual, where they are closely associated 

 with the choroidal mass of tissue that has grown into the eye through the pupil. 



Fig. 8. Exit of Optic Nerve of Eye shown in fig. 7 b. 



