354 



NORMAL HISTOLOGY. 



axis-cylinder processes into the inner reticular layer, to end at 

 various levels in arborizations in relation with the terminal filaments 

 of the ganglion-cells ; their protoplasmic processes extend as far 

 as the outer reticular layer, where they terminate in ramifications 

 beneath the cone-cells. The protoplasmic processes of the rod- 

 bipolars end beneath the rod-cells, their axis-cylinder processes 

 penetrating the inner reticular stratum , to end in close relation with 

 the ganglion-cells. 



The outer reticular layer appears as a narrow zone made up of 

 an intricate net-work of fine fibres with sparingly distributed 

 nuclei. The fibrillse are derived from the neuroglia and from the 

 processes of nerve-cells, among which are the horizontal cells 

 whose axis-cylinder processes extend horizontally within the layer, 

 often for considerable distances, to end beneath the visual cells. 



The outer nuclear layer and the layer of rods and cones, the 

 remaining strata of the inner lamina of the retina, together constitute 

 the neuro-epithelium. Since the rods and the cones and the outer 

 nuclear layer are parts of a single lamina of tall neuro-epithelial 

 elements, the visual cells, of which they are respectively the outer 

 and inner segments, these strata must be regarded as subdivisions of 

 the one broad zone, and not as independent retinal layers. The 

 outer and inner segments are sharply separated by the intervening 

 membrana limitans, through the openings in which the rods and 

 the cones protrude. The constituents of the neuro-epithelium 

 are, therefore, the rod-visual cells and the cone-visual cells, 

 supported by the sustentacular tissue. 



The rod- visual cells are composed of two parts, the one situ- 

 ated without the limitans, including the non-nucleated and highly- 

 specialized segments, the rods, and the other within the limitans, 

 consisting of slender varicose elements, the rod-fibres, provided 

 with fusiform enlargements, the rod-spherules, which contain the 

 nuclei of the visual cells. The rods are slender cylindrical struct- 

 ures, about 60 fj. in length and 2 p. in breadth, composed of two 

 chemically and optically distinct parts, the outer and inner seg- 

 ments. 



The outer segments of the rods are cylindrical, apparently 

 homogeneous, highly-refracting bodies, which, after certain reagents, 

 exhibit a disposition to break up into thin transverse disks. The 

 outer segments of the rods are further distinguished as being the 

 exclusive seat of the peculiar visual purple or rhodopsin. The 

 inner segments of the rods are slightly broader and less regularly 

 cylindrical, and present a finely granular appearance, the parts of 

 the segments nearest the membrana limitans possessing a peripheral 

 longitudinal striation. 



