154 



GENERAL PRINCIPLES OF ZOOLOGY. 



also can be distinguished two kinds of rhabdoms, rods and 

 cones (Fig. 79). 



The Optic Ganglion. 

 Before the optic nerve divides 

 into the separate optic cells, 

 it forms a swelling, the optic 

 ganglion, which either lies 

 as a detached body outside of 

 the eye, or fuses with the re- 

 tina into a connected whole. 

 The parts designated as re- 

 ticular, the internal granular 



Of 



FIG. 78. FIG. 79. 



FIG. 78. Ocellus (oc) of a medusa (Lizzia Koellikeri) with lens (/). 



FIG. 79. Human retina. (After Gegenbaur.) P, pigment-layer; E, layer of sensory cells ; 

 G, optic ganglion ; i, limitans interna ; 2, nerve-fibre layers ; 3, ganglion-cells ; 4, inner 

 reticular layer ; 5, inner granular layer ; 6, outer reticular layer ; 7, outer granular layer ; 

 8, limitans externa ; . ;, rods and cones ; 10, tapetum nigrum ; M, Miiller's fibres. 



layer and the ganglion-cells (nerve-fibre layer) (Fig. 79 G), 

 belong to the optic ganglion ; the layer of the optic cells 

 consists only of the external granular layer and the rods and 

 cones resting upon it (Fig 79 E.~] 



Refractive Bodies in the Eye. The complicated 

 structure of the eye is further influenced by the facts that 

 special refractive bodies (cornea, lens, vitreous body) con- 

 centrate the light in order to cast an image upon the retina \ 



