390 



THE EYE IN EVOLUTION 



L;a?je-T 



W; 



5^'--^.; 







sionally, particularly in the Indian python, Python molurus, choroidal 

 vessels somewhat resembling those seen in the human eye are evident 

 in the periphery of the fundus. The optic disc is always round and 



white, although it varies much in 

 size ; that of the water-snake, 

 Trojpidonotus fasciatus, is enormous, 

 exceeding in size that of any 

 Vertebrate with a circular disc, even 

 that of the whales in which the eye 

 may reach a diameter of 5| inches. 

 Usually on the surface of the disc 

 there is some melanin pigment, some- 

 times in small quantity (Boidse), 

 sometimes associated with a cushion 

 of mesoderm, resembling the ap- 

 pearance seen in Crocodilians 

 (Beauregard, 1876 ; Kopsch, 1892 ; 

 Leplat, 1922 ; Jokl, 1923). This, re- 

 presenting the remains of mesoderm 

 entering with the hyaloid vessels, is 

 functionless and is not homologous 

 with the neuroglial conus of 

 lizards although in certain species 

 it may project into the vitreous to 

 form a very similar structure (pig- 

 mented in the British adder, Vipera 

 berus ; colourless in the king-snake, 

 Lamprojjeltis). The remains of the 

 hyaloid vasculature, however, form 

 a well-defined system of vessels, three 

 and sometimes four of which emerge 

 through the disc from the optic nerve. 

 In some species these are small and 

 are apparent only a short distance 

 from the disc (Boidae) ; more usually 

 arteries of considerable size run 

 nasally and temporally, drain into two 

 venous arcs which encircle the globe in the region of the orbiculus, and 

 combine to form a hyaloid vein which runs backwards in the fundus mid- 

 ventrally to leave the eye at the optic disc. Over the surface of the retina 

 lying in the vitreous there is a membrana vasculosa of very fine capil- 

 laries (Fig. 471) (Hyrtl. 1861 ; Virchow, 1901 ; Szent-Gyorgyi, 1914); 

 onl\- ■■ the colubrid snake, TarbojjJiis, are these known to penetrate 

 the 1- a itself.^ 



' Cf. the direct \-asculai'ization of the retina of the eel, p. 300. 



D 



■-> Vf w 



v< 



Fig. 474.— The Retina or Leptodeira 



A.W PLATA. 



The mixed retina of a nocturnal 

 snake. 1, optic nerve layer ; 2, gan- 

 glion cell layer ; 3, inner plexiforni 

 layer ; 4, inner nuclear layer ; 5, outer 

 plexiform layer ; 6, outer nuclear 

 layer ; 7, external limiting membrane ; 

 8, visual cells (above are rods, and 

 below cones ; D, double cone ; S, 

 single cone) ( X 500) (Gordon Walls). 



