THE LACERTILIAN RETINA 625 



with their surfaces heavily dusted with pigment granules, and among 

 these just enough neuroglial tissue to hold the whole together and in 

 shape. In cross-section the conus may be circular, oval, X- or Y-shaped — 

 in the latter cases, foreshadowing the buttressed 'pecten' of the lower 

 birds. It is supplied by an artery and a vein which reach it through the 

 optic nerve, and is one of the various devices which many vertebrate eyes 

 have found necessary for supplementing the disadvantageously-located 

 chorioid in the nutrition of the inner layers of the retina (see pp. 648-58) . 

 In profile the conus may be stubby, long and nearly cylindrical, or 

 dagger-like. In length, it varies from a nubbin in most nocturnal forms 

 (Xantusia, many geckoes) and sluggish species (skinks) to a third or 

 more of the diameter of the eyeball — in such instances, nearly reaching 

 the lens. It is completely lacking only in the various families of worm- 

 like, burrowing lizards (Amphisbaenidae, Euchirotidae, Anelytropidae, 

 Dibamidae*), whose tiny (less than 1.0mm.) eyes are buried beneath 

 (usually) opaque skin and seldom consist of more than a connective- 

 tissue capsule containing an optic cup and a lens. 



The Lacertilian Retina — The outstanding feature of the retina is its 

 fovea centralis, which is not known to be lacking in any diurnal lizard. 

 The fovea may, as in chameleons, be larger than that of man, and with 

 a vastly greater concentration of visual cells (as many as 756,000/sq. 

 mm.). The fovea is absent, despite statements to the contrary, in all 

 geckoes which have ever been examined; and it is also wholly lacking in 

 pygopodids, Heloderma, and most xantusiids. In Xantusia vigilis, how- 

 ever, just enough trace of an area centralis has survived the family's 

 adoption of nocturnality to enable one to tell where in the retina the 

 fovea used to lie. The disturbance created in the average lizard eye by 

 the fovea scarcely finds scope to subside before the ora terminalis is 

 reached (see chameleon in Fig. 71, p. 173), and it is only in monitors, 

 iguanas, and the like that the retina has sufficient area to boast a wholly 

 unaffected 'extra-macular' peripheral zone of any great width. 



In its laminal purity and in the thickness of its inner-nuclear and 

 ganglion layers, the lacertilian retina is exceeded only (and not greatly) 

 by the visually supreme birds. The pigment-epithelial processes are 



*But not Aniella. The eyes of this Httle Cahfornian worm-lizard are a Httle less than a 

 millimeter in diameter, but they have all their 'works' — pigmented conus, normal retina and 

 visual cells, scleral ossicles, ciliary muscle, ringwulst etc. This genus is erroneously listed 

 in Table XI (p. 450) as having a spectacle. The lids are mobile, despite statements to the 

 contrary in herpetologica! literature. 



