VISUAL SYSTEM: STATE OF THE ART 47 



structures this activity occurs form the discipline of correlative retinal 

 physiology (Stell 1972a). This marriage of biochemistry, physiology, and 

 morphology has produced, in the last 10-15 years, what must be regarded as 

 astounding progress in the field of visual processes. This progress has no 

 doubt been aided by the general advance in biological technology, but that 

 cannot be the whole story. Generous research support combined with the 

 dedication of a number of brilliant workers and development of some highly 

 analytical methods (i.e., microelectrode technique, radioautography, single- 

 cell microspectrophotometry, freeze-etch methods, etc.) have added to the 

 unparalleled growth of our understanding of the retina. The elasmobranchs 

 have become increasingly important experimental subjects during this 

 growth period, as evidenced by the more than 60 publications on the retina 

 and vision of these fishes between 1960 and 1975. 



Retinal Anatomy— Several excellent and comprehensive descriptions 

 of retinal organization in the vertebrates are available (Dowling 1970, Dunn 

 1973, Rodieck 1973, and Stell 1972a) and the reader is referred to these for 

 details. For reference, however, a brief description of the generalized verte- 

 brate retina will be given. 



The sensory retina is a highly ordered tissue arranged into three cellular and 

 two synaptic layers (Dowling 1970). The scleral border of the retina is 

 comprised of a layer of nutritive cuboidal cells, the retinal epithelium. 



The epithelium is separated from the choroid by a basal lamina, Bruch's 

 membrane. In man, Bruch's membrane is a complex pentalammelar structure 

 composed of connective tissue (Rodieck 1973). The retinal epithelium, in 

 addition to nutritive function, is active in visual pigment regeneration and 

 renewal of the outer segments of rod photoreceptors. Fine structure of the 

 elasmobranch pigment epithelium is poorly known; however, one note- 

 worthy feature is that it lacks the characteristic melanin pigment granules 

 found in other vertebrates (Miiller 1856). 



The most distal of the three cellular layers comprising the vertebrate retina 

 is the receptor or bacillary layer containing the rods and cones. This means 

 that light must pass entirely through the retina before impinging on the 

 receptors. 



The majority of synapses between retinal neurons are confined to two 

 synaptic or plexiform layers. The outer plexiform layer is the synaptic site of 

 receptor terminals (rod spherules and cone pedicles) and the dendrites of 

 horizontal and bipolar cells, both of which comprise the inner nuclear layer 

 along with the perikarya (cell bodies) of the amacrine cells. Amacrine and 

 bipolar cells contact ganglion cells in the inner plexiform layer and the 

 ganglion cell bodies along with their axons form the innermost (vitread) 

 optic nerve and ganglion cell layers. 



This basic organization was well known by the 19th century histologists, 

 especially from the work of Ramon y Cajal (1893, etc.). One other major 

 group of cells, known as Miiller or radial fibers, traverses the retina vertically 

 from receptor to ganglion cell layer; they are glial elements which provide 

 support for the neurons. 



