THE LACERTILIAN EYE 623 



135), To make complete the identity between the 'diurnalized' eye of 

 Sphenodon and that of the lizard, we should need only to pluck the 

 droplet-free cones from the Sphenodon retina. 



The adnexa have been described adequately elsewhere (p. 423). The 

 eyeball is as high as it is wide except in the largest lizards, where it has 

 some horizontal ellipsoidality. The axial length is shortened somewhat, 

 and in diurnal species the circumcomeal region of the sclera is more or 

 less concave — supported so by the ca. 14 scleral ossicles, as part of the 

 means by which the ciliary body and the lens are brought into contact 

 for the purposes of accommodation. The thin scleral cartilage usually 

 reaches forward at least to the equator, often beyond, where it is met (or 

 a bit overlapped) by the broad, thin, ossicular ring (Fig. 182, p. 632). 

 In the chameleon, however, the cartilage is reduced to a four-millimeter 

 disc which lies behind the foveal region. 



The circular cornea is usually of uniform thickness throughout its 

 arch (c/. Sphenodon) , and is relatively thin in large eyes, relatively 

 thick in small ones. Its sharp curvature continues for a little way into 

 the ossicular zone, before the sigmoid flexure of the ossicles reverses the 

 curvature to become the more gentle one of the posterior segment. The 

 usual layers are present except in some geckoes, where (e.g., in Hemi- 

 dactylus mabouia) there is no trace of Descemet's membrane or meso- 

 thelium.* At its margin, the corneal substantia propria separates briefly 

 into three laminae, the two outermost becoming the fibrous investment 

 and lining of the ossicular zone of the sclera, while the innermost blends 

 with the connective tissue of the iris-angle region and often serves as the 

 'tendon' of the ciliary muscle. 



Except in the smallest eyes, the retina thins out very gradually toward 

 the ora terminalis (as in Sphenodon) , where the thin chorioid becomes 

 the ciliary body. This is very broad owing to the disparity between the 

 size of the posterior segment and that of the cornea. The base-plate 

 diverges slowly from the sclera, so that even at its anterior end the ciliary 

 body is not very thick (compare Sphenodon; contrast chelonians and 

 crocodilians) . The anteriormost strands of the meshwork tissue of the 

 iris angle may be organized as a pectinate ligament, but this is never as 

 well defined as in birds. There are no ciliary processes. Nevertheless, 

 the ciliary body has a broad zone of firm contact with the lens, which is 



*Present, however, in Coleonyx — as one of the many reasons (ophthalmological ones, at 

 least) for considering this and other 'eublepharid' geckoes to be a distinct group with per- 

 haps only very distant kinship with the spertacled majority of geckoes. 



