44 



BLIND VERTEBRATES AND THEIR EYES. 



The Lens. — The lens has retained its natural shape and position (lens, fig. 14). 

 It is almost spherical and measures 80 /a in diameter. In most of the sections an 

 outer layer of cells extends around the anterior surface of the lens. The interior 

 in nearly every case stained as a structureless mass, but in a few sections it appeared 

 to be made up of large irregularly shaped cells with small nuclei. If any fibrous 

 cells still persisted, they did not show. No capsule is present. 



Fig. 14. Horizontal Section of Eye showing Different Parts. Retina diagrammatic. 

 For explanation of letters see fig. 13. 



The Vitreous Body. -- The vitreous body (vit., fig. 14) occupies the greater part 

 of the eyeball and has certainly undergone but little change. The aqueous cavity 

 has entirely disappeared. 



The Iris. — Only the uveal part of the iris remains. It is continuous with the 

 pigment epithelium of the retina and has the same structure. In the thickest part 

 it measures 68 /*. The cells are similar to those of the pigment layer, except that 

 their radial diameter is much greater. The ciliary processes are no longer present. 



The Optic Nerve. — The optic nerve can be traced from the eye, through and 

 along the side of Harder's gland. While the nerve could be traced no farther on 

 account of an incomplete series of sections, there is no doubt that the connection 

 with the brain still exists. The nerve fibers enter the eye in a compact mass, pass 

 through the layers of the retina until they reach the nerve fiber layer, where they 



