22 VISION 



relates to water imbibition and swelling properties. For example, Maurice 

 and Riley (1970) attribute corneal swelling directly to repulsion of nega- 

 tive charges found on mucopolysaccharide molecules. Anseth (1961) sug- 

 gested that the glucosamine :galactosamine ratio might be critical to the 

 question of water uptake, since it is high in animals that exhibit limited 

 corneal swelling. Because of the importance of mucopolysaccharides in water 

 uptake and because of the unusual swelling properties of elasmobranchs, 

 several authors (Suzuki 1960, Mathews and Inouye 1961, Robert and 

 Schillinger 1967, Moczar et al. 1969, and Praus and Goldman 1970) have 

 investigated these sugar-protein complexes in the cornea. Robert and 

 Schillinger (1967) demonstrated that elasmobranch and teleost corneas were 

 biochemically dissimilar and that the cornea of Scyliorhinus contained much 

 more insoluble protein than those of teleosts. They suggested that the high 

 content of keratoglycosaminoglycan in the shark played a role in maintain- 

 ing transparency. Moczar et al. (1969) came to a similar conclusion in a more 

 detailed study on cod, whiting, and dogfish (Scyliorhinus). They felt that 

 resistance to swelling might be related to insoluble mannose-containing 

 glycoproteins in the stroma. The cornea of the Scyliorhinus had the highest 

 percent dry weight, lowest hydration, most insoluble stroma, and lowest 

 mucopolysaccharide-protein observed in their study. Praus and Goldman 

 (1970) found a significant difference between corneal mucopolysaccharides 

 of mammals and Squalus: galactosaminoglycan (chondroitin sulfate) pre- 

 dominates by 75% over glucosaminoglycan (keratin sulfate) in the shark. 

 Thus, the shark cornea more closely resembles shark cartilage, which con- 

 tains more than 90% chondroitin sulfate. Exactly how chondroitin sulfate is 

 associated with nonswelling properties of the elasmobranch cornea is the 

 subject of future studies by Praus and Goldman. 



The Choroid 



The choroid coat may be considered that part of the uveal tract which lies 

 just internal to and lines the sclera. In most vertebrates it consists of connec- 

 tive, vascular, and pigmented tissue. In elasmobranchs the uvea is the only 

 vascularized tissue in the adult eye. According to Walls (1942) two blood 

 vessels enter the globe: a temporal choroidal artery and a ventral artery 

 supplying the iris. The eye is drained by two main veins, one dorsal and one 

 ventral. Francois and Neetens (1974) briefly summarized the vascular supply 

 of the elasmobranch eye. 



In some species the choroid is thickened by a tangle of blood vessels, 

 connective tissue, and possibly lymphatic spaces known collectively as the 

 suprachoroidea. The entire outer part of the uvea has been called the epi- 

 choroid (Duke-Elder 1958). The inner part, lining the retinal epithelium and 

 supplying the retina with nutrition, is the choriocapillaris. Between lies the 

 unique choroidal tapetum of the elasmobranchs. 



Tapetum— The tapetum lucidum is a specialized ocular structure re- 

 sponsible for the eyeshine of animals. Eyeshine is widely distributed among 



