XIII 



PHYLUM CHORD AT A 



111 



the ciliary processes and the posterior face of the iris. The 

 optic 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. n.), 

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

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

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

 forming the outer surface of the retina proper, a layer of bodies 



FIG. 785. Diagram of the retina, the supporting structures to the left, the nervous and epithelial 

 elements to the right ; a d, fibrous supporting structures ; gr. gr'. granular layers ; n.c, 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 Vertebrata, 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 

 imbedded in a single layer of hexagonal pigment-cells, loaded with 

 granules of the black pigment already referred to. 



Immediately behind and in close contact with the iris is the 

 transparent biconvex lens (Fig. 784, Z.), formed of concentric layers 

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

 a delicate capsule, attached by a suspensory ligament (sp. I.) to the 

 ciliary processes. The suspensory ligament exerts a pull upon the 



