THE CHELONIAN EYE 



609 



The Eye as a Whole — The eyeball has equal vertical and horizontal 

 diameters, and a slightly shorter axial diameter (Fig. 175). Its internal 

 proportions are those of a diurnal eye with a broad retinal image and 

 high resolution. At the same time, its dioptric media are the most trans- 

 parent known — which one would expect to be true of some nocturnal 

 animal. 



The sclera consists largely of a cup of cartilage which reaches forward 

 beyond the equator, from where the zone occupied by the scleral ossicles 

 extends to the corneal rim as a flat-surfaced, truncated cone. The cornea 

 is thick (except in sea turtles?) and bears a relatively thick epithelium 

 and prominent Descemet's layers. The substantia propria, at the limbus, 

 divides into two portions, the inner of which receives the tendinous 



Fig. 175 — Right eye of a turtle, 

 Testudo graca, in horizontal section. 

 Redrawn, modified, from Szent- 

 Gyorgyi. (The lens is shown in full 

 accommodation ) . 



c- chorioid; co- conjunctiva; cs- canal 

 of Schlemm; he- hyaloid canal of 

 vitreous; /- lens; mt- meshwork tissue 

 of iris angle; n- nasal side; o- optic 

 nerve; r- retina; sc- scleral cartilage; 

 so- scleral ossicles; t- temporal side; 

 Z- zonule. 



origins of the ciliary-muscle fibers, while the outer splits to enclose the 

 scleral ossicles and then recombines to pass over the outer surface of the 

 scleral cartilage as the fibrous layer of the sclera. As in reptiles in gen- 

 eral, the boundary between cornea and sclera is indicated by a deposit 

 of pigment in the fibrous tunic. 



The chorioid is of ordinary thickness and is not richly vascular except 

 in marine forms. Anteriorly, where the chorioid merges into the ciliary 

 body, the inner layers of the uveal coat, together with their epithelial 

 (retinal) facing, swing gradually away from the fibrous tunic, leaving a 

 long, sharp cleft to be filled in with loose connective tissue which thus 

 suspends the iris from the limbus corneae. The canal of Schlemm lies 

 against the sclera in this meshwork tissue, in a position approximating 



