XIII 



PHYLUM CH( )RDATA 



111 



the ciliary processes and the ])()st('rior face of the ins. The 

 oy)tic nerve (O.N.) pierces the sclerotic and choroid and becomes 

 continuous with the retina, its fibres spreading over the inner 

 surface of the latter. Microscopic examination shows that these 

 fibres, which form the innermost layer of the retina (Fig. 785, o. m.), 

 turn outwards and become connected with a layer of nerve-cells 

 (n.c). E.xternal to these come other layers of nerve-cells and 

 granules, supported by a framework of delicate fibres, and finally, 

 forming the outer surf;\ce of the retina proper, a layer of bodies 



1___JL_ a' 



nu 



71. c 



b" v.c 



n.c 



o.n 



Fig. 7S5. — Diagram of the retina, the supporting .struct\n-es tu the left, the nervuiisand epithelial 

 elements to the right ; a — i/, fibrous supporting structures ; r/r. (ir'. granular layers ; n.r, -n.c'. 

 n.c"., n.c'". nerve-cells; nu. nuclear layer of rods and cones; o. n. fibres of optic nerve; 

 r. rods and cones. (From Wiedersheim's VerUhrata, after Stohr.) 



which correspond to modified sensory cells and are called, from 

 their shape, the rods and cones (r.). These are placed perpendi- 

 cularly to the surface of the retina, and their outer ends are 

 imbeclded in a single layer of hexagonal pigmrjit-cells, loaded with 

 granules of the black pigment already referred to. 



Immediately behind and in close contact with the iris is the 

 transparent biconvex lots (Fig. 784, L.), formed of concentric layers 

 of fibres each derived from a single cell. The lens is enclosed in 

 a delicate capsule, attached by a su&pcnsorij ligament (sp. I.) to the 

 ciliary processes. The suspensory ligament exerts a pull upon the 



