THE INTERNAL OR NERVOUS TUNIC OF THE EVE. 459 



light and dark (striation of the rod-nuclei). This striation is not gen 

 erally observed in the rod-nuclei of the human retina. 



The cone=visual cells consist, similarly to the rod-visual cells, 

 of a cone and a cone-fiber with its nucleus. The cone (i5// to 

 25 (i in length) is, as a whole, shorter than the rod, and its inner 

 segment is considerably broader than that of the rod. The cone 

 ellipsoid comprises the outer two-thirds of the inner segment, and 

 the outer segment has a more conical shape. The cone-fiber like- 

 wise extends as far as the outer molecular layer, where it ends in 

 a branched basal plate. Its somewhat larger nucleus is always 

 found in the vicinity of the inner segment of the cone. The 

 inner surfaces of the inner segments, not only of the cone-cells, but 

 also of the rod-visual cells, lie in one plane, corresponding to the 

 external limiting membrane, a structure composed of the sustenta- 

 cular fibers of .Muller. The rod-fibers and cone-fibers, with the 

 nuclei of the rod- and cone-visual cells, lie between the external 

 limiting membrane and the outer molecular layer. It will be 

 observed, therefore, that the visual cells include the layer of rods 

 and cones and the outer nuclear layer. 



The outer molecular layer consists : (i) of the ramifications of 

 Miiller's fibers ; (2) of the knob and tuft-like endings of the visual 

 cells ; and (3) of the dendritic processes of the bipolar cells of 

 the inner nuclear layer. These structures will be considered more 

 in detail in discussing the relations of the elements comprising the 

 retina. 



The inner nuclear layer contains: (i) the nucleated stratum 

 of Miiller's sustentacular fibers ; (2) ganglion cells situated in the 

 outer region of the layer and extending in a horizontal direction ; 

 (3) bipolar ganglion cells with oval nuclei, densely placed at various 

 depths of the layer and vertical to it ; (4) amacrine cells (neurones, 

 apparently without neuraxes) lying close to the inner margin of 

 the layer and forming with their larger nuclei a nearly continuous 

 layer of so : called spongioblasts. The numerous processes of these 

 spongioblasts lie in the inner molecular layer, the composition of 

 which will be further discussed later. 



The ganglion-cell layer of the optic nerve consists, aside from 

 centrifugal neuraxes and the fibers of Muller, which are here 

 present, of multipolar ganglion cells, the dendrites of which extend 

 outward and the neuraxes of which are directed toward the optic 

 nerve-fiber layer. These cells vary in size, and their nuclei are 

 typical, being relatively large, deficient in chromatin, and always 

 provided with large, distinct nucleoli. In man the optic nerve- 

 fibers of the retina are nonmedullated. 



All these structures are typical of that portion of the retina 

 lying behind the ora serrata. The retina in the vicinity of the optic 

 papilla and macula lutea must be taken up separately. 



