VISUAL SYSTEM: STATE OF THE ART 21 



Smelser (1962) showed that the naked stroma of Mustelus loses up to 15% 

 of its weight (mostly water) when immersed in distilled water for 1 h. The 

 stroma of the scup, Stenototnus, a marine teleost, swells by 350% under the 

 same conditions. Obenberger et al. (1971a) investigated details of corneal 

 hydration in Scyliorhinus. They confirmed that the elasmobranch cornea 

 actually loses weight when immersed in a variety of liquids. They also ob- 

 served marked solubilization of solid components, the cornea dissolving up 

 to 22% in distilled water. In a second study, Obenberger et al. (1971b) found 

 that the cornea could be made to swell in media of low pH. Paradoxically, a 

 pH of 4 is the point of minimal swelling in mammals. 



Tolpin et al. (1969) investigated the relation between swelling pressure 

 (force per unit area needed to maintain constant corneal thickness) and 

 hydration of the cornea in Squalus. The normal value of corneal hydration 

 (3.2 mg H2 0/mg dry wt of the cornea) does not differ from that of 

 mammals. The main difference is that normal swelling pressure for Squalus is 

 0.0 mm Hg while the normal value for mammals is 50 mm Hg. This means 

 that at maximal swelling, forces in the dogfish cornea are exactly balanced 

 (presumably) by the sutural fibers. In contrast, the mammalian cornea im- 

 bibes up to 12 times the normal value of water before swelling ceases. These 

 results confirm that a corneal fluid transport mechanism, important in mam- 

 mals, need not operate in sharks. 



Physiological investigations by Edelhauser (1968) provided data on the 

 passage of water and salts through the cornea. According to Edelhauser, the 

 aquatic cornea is devoid of a tear film and thus the problem of water and ion 

 passage across this tissue can be critical (but see Harding et al. 1974). The 

 problem of water and ion flow is intimately bound up with the adaptation of 

 elasmobranchs to life in a salty medium. It is well known that elasmobranchs 

 have achieved "osmotic superiority" in the marine environment by storing 

 huge amounts of urea and trimethylamine (oxide) in their body. Their en- 

 vironmental situation is thus somewhat analogous to that of freshwater fish: 

 the external medium in which sharks live is relatively hypotonic to their 

 internal medium. 



Edelhauser found the cornea resistant to water and sodium influx from 

 the environment. No net movement of radioactive sodium (Na 23 ) or triti- 

 ated water (H^O) across the cornea was observed, regardless of osmotic 

 gradient. The thick epithelial layer of the cornea appeared to offer the 

 greatest resistance to passage of materials. Impermeability to sodium and 

 water is typical of the aquatic cornea; the aerial cornea is permeable to these 

 materials. Harding et al. (1974) reported the existence of a viscous pre- 

 corneal film, presumably in elasmobranchs and certainly in teleosts. Some of 

 the resistance to transport across the cornea could reside in this coating. 



Biochemistry— The chemical composition and biochemical reactions 

 in the cornea do not differ significantly from those of other connective tis- 

 sue. However, the mucopolysaccharide content does form a cornea-specific 

 pattern (Maurice and Riley 1970). The importance of corneal glycoproteins 



