16 VISION 



they tend to destroy stabilization of the visual field produced by normal eye 

 and head movements. 



The Whole Eye— The eyes of most elasmobranchs are prominent and 

 are placed laterally on the head and set for some degree of binocular overlap. 

 One exception may be the hammerheads (Sphyrnidae) whose visual fields 

 apparently do not overlap (Walls 1942). The size of the globe varies from less 

 than 1% of total length in the Orectolobidae to several percent in the deep 

 sea squaloids. Sharks with completely degenerate eyes are unknown. 



Figure 2, the eye of Negaprion sectioned in horizontal and vertical plane, 

 demonstrates that the globe of sharks is not spherical; it is strongly ellip- 

 soidal, being most compressed in the anteroposterior axis. 



Details of the whole eye relative to image formation and optical land- 

 marks are covered elsewhere in this volume. 



The Cornea 



Light enters the eye through the cornea, a more structurally organized con- 

 tinuation of the fibrous outer layer, the sclera. In addition to its optical 

 properties, the cornea must withstand intraocular pressure from within and 

 protect the eye from without. It is distinguished from other ocular tissues by 

 its anatomical position. The cornea is the interface between eye and environ- 

 ment, with all the concurrent problems of water and ion flow, but, since it 

 must remain transparent, blood vessels are absent. This poses a distinct prob- 

 lem for nutrition (Maurice and Riley 1970). 



In several vertebrate classes the cornea is modified, for example, as an 

 ocular filter. In contrast, the cornea of elasmobranchs is structurally and 

 optically simple. While it possesses all the usual vertebrate layers, including 

 Descemet's membrane (not usually found in teleosts), one attribute renders 

 this cornea unique among vertebrates: it does not swell in distilled water. 

 This is unexpected, not only because most other corneas swell but also 

 because of the high osmotic pressure in elasmobranch tissue. This simple 

 property, long ago recognized by Ranvier (1878), means the elasmobranch 

 cornea remains transparent under a variety of conditions. Clinically, the 

 properties of low water uptake and resistance to opacity make the elasmo- 

 branch cornea ideal for use in heterograft transplants. Payrau (1965, 1969) 

 reported that this cornea is well tolerated by hosts, and several shark-human 

 transplants have been made. Actually, the elasmobranch cornea differs from 

 other vertebrate corneas in many interesting anatomical, biochemical, and 

 physiological features (Obenberger et al. 1971a). 



Anatomy— Smelser's (1962) redescription of the nonswelling prop- 

 erties of the elasmobranch cornea created renewed interest in the structure 

 of this tissue. Faure (1970), reporting on the embryonic development of the 

 cornea in Scyliorhinus, made detailed observation at the optical and electron 

 microscope level on three growth stages: the 30-mm and 75-mm embryo and 

 the 140-mm (5-m) "young dogfish." In the 30-mm stage the completely 

 ectodermal cornea is an acellular secretion of the epithelium. Early on, the 



