362 



THE EYE IN EVOLUTION 



while in Conolophus cristatus there are yellow spots over the green 

 background. Usually the semi-opaque nerve fibres radiate uniformly 

 outwards from the disc, sometimes, as in the American " glass-snake," 

 O'phisatirus ventralis, coarse in texture, sometimes so fine as to be barely 

 visible (the leaf-footed lizard, Pygojnis lejndopus, Cham.celeo7i). The 

 disc itself is circular and white but is practically entirely obscured by the 

 conus. The retina is invariably entirely avascular. 



Fig. 429. — The Posterior Pole of the Eye of the Lizard, Lacerta muralis. 



Showing the optic nerve and the conus papillaris approaching the lens ( X 50) 

 (Katharine Tansley). 



Nutrition is conveyed to the retina by a peculiar vascular structure, 

 the CONUS PAPILLARIS, an outgrowth of glial tissue from the optic disc 

 supplied by an artery and vein issuing from the optic nerve and derived 

 from the hyaloid (not the choroidal) vascular system (Fig. 429). 

 Originally described by 8oemmerring (1818) in the eye of lizards {Lacerta 

 monitor, L. vulgaris, L. iguana), the conus has attracted a great deal of 

 study. ^ It is a richly vascular structure with a central artery and vein 

 surrounded by a thick layer of wide capillaries heavily dusted with 

 pigment granules, the A\'hole lying in a framework of neuroglial tissue 



' i i- (1853), Hulke (1864), H. Mliller (1862), Beauregard (1876), Kopsch 



(1892), - ow (1901), Jokl (1923), Johnson (1927), and many others. 



