HISTORY OF THE OPHIDIAN EYE 635 



Harderian gland ballooned, as it has in caecilians and blind lizards; 

 and — as also in these forms — the lacrimal gland disappeared. 



The loss of the fovea and the simplification of the retina involved 

 the optic nerve, which became slender and lost its septa; and the ecto- 

 dermal conus papillaris vanished along with the need for it. The pupil 

 lost all mobility as the iris muscles disappeared, and the chorioid and 

 sclera coalesced as in rudimentary eyes in general. The canal of Schlemm 

 shrivelled; and the eye finally 'touched bottom' in a condition not much 

 if any better than that of a modem Typhlops. Indeed, the organization 

 of the Typhlops eye is such that this worm-like form could well have 

 been the 'first' snake (see Plate I). 



How long the snakes lived underground, no one can say; but they 

 did not (or did not all) remain there. Coming back to the surface, they 

 were under the necessity of reconstituting their eyes almost 'from scratch'. 

 The vestigial nubbins of visual cells had to be nurtured into bloom as 

 respectable rods. Then, as the race became better able to stand the light, 

 the retina became duplex. The eye enlarged, but in the absence of stiff- 

 ening structures in the sclera it was forced to remain forever spherical. 

 With the ciliary body and the lens now far out of contact, an entirely 

 novel means of accommodation had to be devised. The remnants of 

 ciliary muscle moved into the iris to play a dual role in accommodation 

 and in the operation of the (slit?) pupil. 



The recrudescent retina demanded a better nutrition than the thinned 

 chorioid alone could supply. So, the snakes developed a mesodermal 

 conus papillaris, but shortly abandoned this in favor of the more 'direct' 

 vitreal vessels (a change from which the lizards — and the birds — would 

 probably profit if they could make it; see pp. 653-4). They also pro- 

 duced a new 'canal of Schlemm', in a new location and with new con- 

 nections to the venous system, and elaborated a whole new circulation 

 for the iris (which, if we can go by the caecilians, probably had no meso- 

 derm left in it during the underground period). And when (in the Col- 

 ubridae) the retina finally became pure-cone, with new and unique double 

 elements, supporting a diurnality as thoroughgoing as that of any lizard, 

 the needed yellow filter was manufactured out of the lens itself. With 

 a high ratio of optic nerve fibers to visual cells once more restored, the 

 optic nerve became too plump to remain a simple cord, and an entirely 

 new system of fasciculation and septation was invented for it. 



The resulting eye — as we see it today — presents substitutes for all 

 the losses, remedies for all the defects, of the vestigial organ of the 



