THE EYE AND ITS APPENDAGES. 355 



The inner segments of the rod-visual cells include the rod- 

 fibres and their nucleated expansions, the rod-granules. The 

 rod-fibres are slender, greatly extended, and often varicose, and 

 reach from the membrana limitans to the external border of the 

 outer reticular layer. Each rod-fibre represents the greatly attenu- 

 ated protoplasmic body of a visual cell, the situation of whose 

 nucleus is indicated by the ellipsoidal enlargement. These enlarge- 

 ments, the rod-granules, vary in position, sometimes lying near the 

 outer end, at other times close to the middle or the inner extremity 

 of the fibres. The granules are almost entirely occupied by the 

 nuclei of the visual cells, which are covered by an extremely thin 

 layer of the cell-protoplasm. The nuclei of the cells are oval in form, 

 about 6 At in length, and characterized by a remarkable differentiation 

 of their substance into lighter and darker transverse bands. 



The cone-visual cells consist also of two parts, the outer di- 

 visions, the cones, situated beyond the membrana limitans, and the 

 inner portions including the cone-fibres and their nucleated cone- 

 granules. The cones, like the rods, present inner and outer seg- 

 ments, which in physical and chemical properties resemble the 

 corresponding parts of the rods ; the cones, however, are little more 

 than half (3236 /a) the length of the rods. 



The inner segments of the cones are much wider than their 

 outer divisions, and appear as truncated conical bodies whose 

 sides are not absolutely straight, but slightly convex. The outer 

 part of these segments is occupied by an ellipsoidal group of fine 

 longitudinal fibrillae, the fibre-body, which corresponds with the 

 similar structure sometimes present within the rods. 



The inner segments of the cone-visual cells, representing 

 the bodies of the elongated cells, include the cone-fibres and their 

 granules. The cone-fibres differ from the rod-fibres in being 

 broader at the outer ends and more regular in their general contour ; 

 the cone-granules always lie, except in the macular region, next the 

 membrana limitans. 



The distribution of the two kinds of visual cells varies in the 

 different retinal regions ; the arrangement prevailing throughout the 

 greater part of the retina is such that the adjacent cones are separated 

 by three or four rods, the latter far outnumbering the cones. On 

 approaching the macula the number of cones increases, the 

 cones being so closely placed that they are separated by only a single 

 row of rods ; within the fovea itself the rods entirely disappear, 

 the entire percipient layer being composed of cones alone. On 

 the other hand, towards the periphery the number of these visual 

 cells diminishes, and at the ora serrata the cones are widely 

 separated, while the relative number of rods is very large. The 



