GENERAL ORGANOLOGT. 



129 



The Eye is in all animals recognized by the character of the 

 sensory epithelium, the retina. This always has a large amount 

 of pigment which lies either in the sensory cells or in special cells 

 arranged between or behind them. The simplest-formed eye, 

 therefore, appears as a sharply circumscribed pigment-spot in the 

 epithelium of the skin, provided with nerves, commonly also with 

 a lens (fig. 81). 



Rods and Cones. The sensory cell itself bears usually at its 

 peripheral end a process, the rhabdom. This is a cuticular struc- 

 ture, probably serving to collect the 

 rays of light and thus to stimulate the 

 cell, and has, particularly in the verte- 

 brates, a complicated structure, each 

 rhabdom consisting of an inner and an 

 outer portion. Here can be frequently 

 distinguished two kinds of rhabdoms, 

 rods and cones (fig. 82). 



The Optic Ganglion. Before the 

 optic nerve divides into the separate 

 visual cells it forms a swelling, the 



V 



FIG. 81. Flo 82> 



FIG. 81. Ocellus (oc) of a medusa (Lizzia Koellikeri) with lens (0. 



FlG n !!fc I lman - retina V (After Gegenbaur.) P, pigment-layer; E, layer of sensory 



; G, optic ganglion; 1, limitans mterna; 2, nerve-fibre layers; 3, ganglion- 



cells; 4, inner reticular layer; 5, inner granular layer; 6, outer reticular layer- 7. 



V Muller^^bres 761 ' 5 8 ' limitans externa ? 9 i r ds and cones; 10, tapetum nigrum; 



optic ganglion, which either lies as a detached body outside of the 

 eye, or is united with the retina into a connected whole. The 



