306 



THE EYE IN EVOLUTION 



Figs. 344 and 345. — The Ophthalmoscopic Appearance of the Fundus 

 or Teleostean Fishes. 



Fig. 344. — The cod, Gadus, showing the 

 vessels of the falciform j^i'ocess running 

 over the elongated optic disc and 

 breaking up into 6 branches of the 

 hyaloid artery (after Beauregard). 



Fig. 345.— The scorjjion fish, Scorpcena, 

 showing the optic nerve entrance in 

 relation to the falciform process and the 

 peculiar mosaic arrangement of the back- 

 ground of the fundus (after Franz). 



These illustrations may seem to require an apology but the fundus of a 

 fish is very difficult to see ophthalmoscopically. It can be examined out of 

 water if the fish be kept alive by a current of water supplied to the mouth and 

 gills ; some species such as the carp can survive being kept out of water for 

 some time. The difficulties do not end here. Out of water the cornea is 

 irregular ; and in addition to the great liypermetropia in air of an eye optically 

 designed for vision under water, the splierical shape of the crystalline lens 

 makes the dioptrics such that only a minute portion of the fundus can be seen 

 at one time and no overall view can be obtained. 



'tf"^^' 



. 1 



^ 2 

 3 



■5 n 





^m 7 



8 

 9 



Fig. 346. — The Retina of the Trout, Salmo trutta. 

 1, optic nerve fibre layer ; 2, ganglion cell layer ; 3, inner plexiform 

 layer ; 4, inner nuclear layer with a prominent layer of large horizontal 

 cells (5) ; 6, outer plexiform layer ; 7, outer nuclear layer ; 8, external 

 limiting membrane ; 9, visual cells ; 10, rods ensheathed in pigment (light- 

 adaptc'l) (Azan ; X 112) (Katharine Tansley). 



