18 



VISION 



In a detailed anatomical treatment, Goldman and Benedek (1967) de- 

 scribed the organization of the cornea in Squalus and agreed in principle 

 with Ranvier's observation that the nonswelling properties could be com- 

 pletely understood by the anatomical arrangement of "sutural fibers" run- 

 ning from Bowman's membrane to the posterior surface of the cornea. Gold- 

 man and Benedek stated that of all vertebrates possessing corneas with 

 lamellated stromata the elasmobranchs have the most primitive. The elasmo- 

 branch cornea is, however, clearly advanced over the condition found in the 

 cyclostomes. 



Six specific layers can be recognized in the cornea of Squalus: the epi- 

 thelium, in contact with sea water; the basal lamina; Bowman's layer; the 

 substantia propria or stroma; Descemet's membrane; and a monolayer of 

 mesothelial cells in contact with the aqueous humor (Figure 3). The cornea 

 of man is similarly layered. Tolpin et al. (1969) give the corneal thickness in 

 Squalus as 0.25 mm, increasing by about 30% toward the periphery. Harding 



LIGHT 



corneal 

 surface film 



35% 



corneal 

 epithelium 



basal lamina 



25 mm 



15 %- C?, 



Bowman's 

 membrane 



keratocyte 

 stroma 



sutural complex 

 stromal lamella 



Descement's' 

 membrane 



endothelium 



thick sutural 

 fiber 



LIGHT 



Figure 3 Main features of the elasmobranch cornea. The layer- 

 ing is typical but the proportions (%) differ from those in other 

 vertebrates. The most noteworthy feature is the sutural complex 

 that maintains corneal transparency under harsh conditions. The 

 diagram, derived from several sources, is a composite of the 

 corneas of Scyliorhinus and Squalus. 



