62 VISION 



strated to be in the isthmo-optic nucleus of the brain (see Cowan 1970, 

 review). Thus it is possible that these fibers represent inhibitory pathways 

 from brain to retina, required to suppress information flow from one eye so 

 that the shark can, for example, attend to signals from the contralateral eye. 



Ganglion Cells— The most proximal retinal layer contains the ganglion 

 cells whose axons form the optic tract and thus communicate with the brain. 

 Ganglion cells of elasmobranchs, known since the work of Retzius (1896), 

 have only recently been treated in any detail (Shibkova 1971, Stell and 

 Witkovsky 1973a). Some ultrastructural features of the ganglion cells of 

 Rhinobatos have been given by Dunn (1973). As with other retinal neurons, 

 ganglion cells can be divided into several subgroups, typically characterized 

 by size, location, and dendritic arborization. Polyak (1941) recognized no 

 less than six types of ganglion cells in man. Stell and Witkovsky (1973a) 

 distinguished several ganglion cell types but described only those designated 

 as giant ganglion cells (GGC). Shibkova (1971) reported that GGC's make up 

 only a few percent of the neurons of the ganglion cell layers of the retinas of 

 Squalus and Raja. She estimated the ratio of GGC: medium ganglion cells: 

 small ganglion cells as 1:3:50. 



Morphological characteristics of the GGC's of Mustelus include large, 

 flattened, stellate perikarya (approximately 10 X 40 jum); nonstratified 

 dendritic arbor with simple radiate patterns; and dendritic spread up to 2 

 mm. The axons that form the optic nerve arise from axon hillocks, run for a 

 short distance and become completely myelineated, each finally joining 

 several other axons to course as a bundle toward the optic disk. Although 

 the ganglion cell axons of sharks (and teleosts) are myelinated, those of man 

 and other mammals are usually unmyelinated (Sjostrand and Nilsson 1964); 

 both myelinated and unmyelinated fibers have been reported from the turtle 

 retina (Dunn 1973). 



Stell and Witkovsky divided GGC's into three subgroups, depending on 

 the retinal position of the cell body: (1) ordinary GGC's located at the 

 vitreal side of the inner plexiform layer, (2) displaced GGC's located at the 

 scleral side of the inner plexiform layer, and (3) intermediate GGC's found 

 entirely within the inner plexiform layer. The authors present reasonable 

 arguments that GGC's constitute a distinct class of neurons and that the 

 basis for subdivision into ordinary and displaced GGC's is real. Anctil and Ali 

 (1974) confirmed the presence of these three types of GGC's, along with 

 smaller ganglion cells and "glial" cells in the retina of the hammerhead 

 Sphyrna lewini. 



Dendrites of ordinary GGC's appear to receive input from amacrine and 

 bipolar cells with narrow horizontal spread, while the larger GGC's receive 

 contacts from neurons with greater horizontal spreads. To distinguish pat- 

 terns of ganglion cell organization and to characterize the dendritic arbor, 

 Stell and Witkovsky counted the number of dendritic branches (Figure 15) 

 at various distances from the perikarya of a number of displaced and ordi- 

 nary GGC's. They also diagrammed the extent and form of dendritic fields as 

 a function of retinal location. Results of these studies indicated that the 



