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 



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 ce'.ls ; 

 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 ; r, rods and cones ; 10, tapetum nigrum ; ./)/, Muller'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; 



