582 



THE EYE 



Both segments, but especially the inner, under favorable con- 

 ditions present slight longitudinal striations. These striations, 

 when present, are most distinct in the outer half of the inner rod 



FIG. 427. ISOLATED ROD AND CONE CELLS OF THE PIG. 



a-e, cones ; /-i, rods ; za, outer portion, si, inner portion of the cone, the latter con- 

 sisting of an ellipsoid part, se, and a more or less elongated neck p:irt, m ; sk, cone nu- 

 cleus ; zf, cone fibre ; sa, outer, and si, inner portion of the rod ; sfc, rod nucleus ; .<?/, rod 

 fibre. (After Kolliker.) 



segment. The outer filamentous portion of each rod sometimes 

 exhibits transverse markings, possibly indicating a minute struc- 

 ture which is comparable to a series of superposed disks. 



The inner or nucleated portion of each rod, the rod fibre, is 

 found in the outer nuclear layer and is continued as a fine filament, 

 which, having penetrated the external limiting membrane, extends 

 as far as the border line between the outer nuclear and outer reticu- 

 lar layers, at which level the rod filament ends in a knob-like ex- 

 pansion. At some point in its course through the nuclear layer 

 the rod fibre presents a nucleated enlargement, which, under some 

 conditions, shows one or two alternate light and dark transverse 

 striations. The nuclei of the rod fibres are placed at various levels 

 in the nuclear layer, and collectively occupy nearly its entire thick- 

 ness. Its outer border, however, contains relatively few rod nuclei. 



The cones resemble the rods in structure, but their cytoplasmic 

 segment is shorter and several times as broad. The outer aniso- 



