THE OPHIDIAN EYE 627 



vision. In Aniella and Heloderma the droplets are present though color- 

 less — which is true also of Xantusia, whose outer segments are rod-like 

 in size and shape (Fig. 180b). The geckoes have finished the job of 

 changing the ancestral cones into rods, whose outer segments contain an 

 abundance of rhodopsin and are either very long and slim as in 

 Coleonyx (Fig. 180c), or short and thick as in the spectacled geckoes. 

 A few genera of geckoes have secondarily (or tertiarily!) reverted to 

 round pupils and partial or perfect diurnality. These include Phelsuma, 

 Lygoddctylus, Pristurus, Gonatodes, Microscalabotes, Sphcerodactylus 

 (some spp.), and perhaps Teratolepis. All of these are candidates for 

 histological examination, which in some of them at least will unques- 

 tionably reveal that the visual cells have become 'cones' once more. 



Snakes — Leaving out of account the 'blind' families (Typhlopidae and 

 Leptotyphlopidse) , in which the eye is tiny and vestigial, the eyes of 

 snakes are quite thoroughly standardized in structure. From genus to 

 genus (usually without regard to family boundaries) only minor vari- 

 ations occur, the most important of these being in the structure of the 

 retina, in the shape of the pupil, and in the relative size of the lens — 

 variations which, in short, are the bases of simple differences in visual 

 habits with respect to light intensity. 



The presence of the spectacle cannot be held accountable for the 

 peculiarities of the eyeball other than the thin-ness of the corneal epithe- 

 lium. And these peculiarities are numerous and great: as the ensuing 

 description of the eyeball unfolds, the student who has just read the 

 preceding portions of this chapter will not recognize the snake eye as a 

 'reptilian' one at all; but, under a subsequent heading, an explanation 

 of the unique ophidian pattern will be offered which, it is believed, will 

 be entirely satisfactory. 



The eyeball in life is spherical or even a trifle elongated axially. The 

 sclera is composed entirely of tendinous connective tissue. It is thickest 

 posteriorly, where, in average-sized eyes, it about equals the retina. In 

 the largest snakes (large boas, pythons) it is still thicker — up to one and 

 one-half times the thickness of the retina (e.g., Epicrates) . In very small 

 eyes, however, the sclera is usually very thin. The equatorial zone, where 

 the eyeball wall deforms most during accommodation, is almost always 

 (exception: Acanthophis) the thinnest portion of the sclera. It begins 

 to thicken again, as one passes forward, about at the ora terminalis, in 

 front of which it is quite thick for a little space, then thinned again at 

 its junction with the cornea. The outer surface of the sclera is usually 



