130 



BLIND VERTEBRATES AND THEIR EYES. 



The choroid, in so far as this layer can be distinguished from the sclera, consists 

 of a dense layer of fibers closely applied to the eye. Over the distal surface it is 

 split into two layers between which there are a greater or smaller number of pig- 

 ment masses (fig. 45 b, ch.). These would prove effective to prevent the performance 

 of the natural function of the eye were it functional. Pigment cells are much more 

 sparingly found in other parts of the choroid. Blood-vessels are very few in number, 

 a condition to be expected in such a minute organ. This layer was mistaken for 

 the sclera by Kohl. 



The eye proper of Amblyopsis differs very greatly in different individuals, 

 but in general it maintains a certain degree of development from which the many 

 individual variations radiate. The eye of Troglichthys roses, has similarly a 

 general type of structure which is maintained, but with many variations. This 

 type is more degenerate than that of either Amblyopsis or Typhlichthys subterraneus . 



Fig. 46. Two Horizontal Sections through Eye, showing Extent of Scleral (scL) Curtilages and Tendons of 

 Oblique (a, msc.) and Rectus Muscles (b, msc.r). Fig. a represents section just above Fig. b, from an 

 individual 34 mm. long. Drawn under magnification of 560 diameters. 



The eye of Troglichthys has been derived from an eye like that of Amblyopsis by 

 the disappearance of pigment from the posterior part of the retina and the reduc- 

 tion of the central mass of ganglionic cells to the vanishing point. In the most 

 highly developed eye of T. roscz (o, fig. 47) I found but three of these cells. Both 

 in size and in structure the eye of T. rosa is the most rudimentary of vertebrate 

 eyes so far known, except that of Ipnops which is said to have vanished. 



The vitreous cavity and the hyaloid membrane have vanished. The eye has 

 collapsed, the margins of the iris have probably fused, and the pigmented and 

 inner layers of the iris separated from each other. With this general sketch the 

 elements of the eye may be taken up in detail. 



The pigment layer is variously developed (1 in figs. 46 and 47) and may be quite 

 different on the two sides of the head. One peculiarity is practically always present 

 and very striking. The layer forms a covering over the distal face of the eye where, a 

 priori, there ought to be no pigment, and is thinnest or absent over the proximal face 



