In the most recent review of vision in sharks, Gilbert (1963b) -nade 

 no mention of photochemistry, neurophysiology or psychophysics, reflecting 

 the limited information available at that time. Lately, knowledge of the 

 visual system has rapidly increased as a result of research in diverse 

 biological disciplines. Typically, experimental studies center on 

 questions such as: what is the structure of the visual system; how do the 

 component parts work; and what is the biological function or importance of 

 the system. The significance of these questions relates to elucidation of 

 physiological mechanisms of vision and their behavioral correlates. 



a. Anatomy 



Recent advances have been made in the areas of: 1) retinal history, 

 both at the light and electron microscope level (Hamasaki and Gruber, 1965; 

 Gruber, Gulley and Brandon, in press; Stell, 1973); 2) retino-tectal pro- 

 jections and the central visual pathway (Ebbesson, 1972; Graeber, Ebbesson 

 and Jane, 1973 - see page 21) ; 3) pineal apparatus (Rudeberg, 1969; 

 Hamasaki and Streck, 1971; Gruber et a_l . , 1975); and 4) tapetum and iris 

 (Denton and Nichol, 1964; Kuchnow and Gilbert, 1967; Wang, 1969). Thus, 

 the structure of the visual system, already reasonably well known, con- 

 tinues to be profitably investigated. Two areas of future study on the 

 structure of the visual system could produce useful data: 



1) An ultrastructural investigation of receptor cytoarchitecture 

 and the synaptic relations within the retina with emphasis 



on the outer plexiform layer. Such a study would aid in 

 interpretation of neurophysiological results and visual data 

 in general. 



2) The function of the eye as an optical system including 

 refraction, cardinal planes, nodal points, accommodation, 

 preretinal absorption and spectral reflection from the tapetum. 

 These data could be utilized in producing a schematic or 

 model eye. Thus, levels of photic stimuli impinging on the 

 cornea could be specified at the receptor level. In addition, 

 the physical limitations of the dioptric parts of the eye 

 would be known. 



b. Physiology 



Areas encompassed within visual physiology include: 1) vegetative 

 physiology of the eye such as nutrition of the lens, cornea, iris and 

 retina; 2) photobiology, including identification of visual pigments, 

 kinetics of bleaching and photoproducts; 3) neurophysiology, including 

 electroretinograms (ERG's), intercellular records such as S-potential, 

 intracellular or single unit records and resultant data on frequency 

 codes, receptive fields, and visual mechanisms; 4) psychophysics, in- 

 cluding conditioned responses and natural orientations providing informa- 

 tion about visual capabilities, stimulus limits and visual mechansisms 

 involved with behavioral functions. 



While more is known about the neurophysiology of vision in elasmo- 

 branchs than many other organisms, these studies are still in their 

 initial stages. Several electroretinographic studies have been accom- 

 plished (Dowling and Ripps, 1970; Green and Sigal, 1973; Gruber, 1973; 



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