1464 



HUMAN ANATOMY. 



is composed of an outer and an inner segment, of about equal length. The outer 

 segment possesses a uniform diameter, is doubly refracting, and readily breaks up into 

 minute disks. It is invested with a delicate covering of neurokeratin, contains 

 myeloid (Ktihne) and is the situation of the visual purple or rhodopsin. The inner 

 rod segment is somewhat thicker and has an ellipsoidal form. It is singly refracting, 

 homogeneous in structure (rapidly becoming granular after death) and from its innei 

 extremity sends the delicate rod-fibre through the external limiting membrane into 

 the outer nuclear layer where the nucleus of the rod visual cell is found. 



The cone visual cell is composed of the same general divisions as the rod-cell, 

 including the specialized outer part, the cone, and the body within the external nu- 

 clear layer. The cones are shorter than the rods, and, except in the fovea, have a 

 length of .035 mm. Each one (Fig. 1221) is composed of an outer narrow cone- 

 shaped segment, and an inner broader segment, which is distinctly ellipsoidal in 

 form, with a diameter of .007 mm. The inner segment is double the length of the 



outer, and is continued inward as 



FIG. 1220. t h e cone -jib re with its nucleus in 



the outer nuclear layer. In the 

 fovea, where the cones alone are 

 found, they are of approximately 

 the same length as the rods, and 

 possess about one half the usual 

 diameter. 



The outer nuclear layer, 

 the inner portion of the neuroepi- 

 thelial layer, is composed of the 

 bodies of the rod and cone visual 

 cells, which show chiefly as the 

 nuclei, the so-called rod- and cone- 

 granules. The rod-granules oc- 

 cupy an elliptical enlargement of 

 the attenuated rod-fibres. They 

 exhibit a transverse striation and 

 are placed at varying levels within 

 the layer. The rod-fibres are con- 

 tinued as a thin protopla'smic pro- 

 cess into the outer reticular layer, 

 where they form small end-knobs 

 which are associated with the outer 

 terminals of the small nerve-cells, 

 the rod-bipolars. The cone-gran- 

 ules are less numerous than those 

 of the rods, display no transverse 

 markings, and are found only in the 

 outer portion of the nuclear layer, 

 near the external limiting mem- 

 brane. The cone-fibres, the attenuated bodies of the cone visual cells, are broader 

 than the corresponding parts of the rods and are continued through the outer nuclear 

 layer as far as the outer portion of the external plexiform layer, where they end with 

 a broad base, from which delicate processes extend inward to interlace with the 

 terminal arborizations of the cone-bipolars. The outer nuclear layer is about .05 

 mm. in thickness. 



The outer plexiform layer is a narrow granular looking stratum, between the 

 outer and the inner nuclear layer, and constitutes the first of the cerebral layers 

 of the retina. It is composed of the dendritic arborixations of the bipolar nerve-cells 

 of the succeeding layer, which lie in close relation with the centrally directed proces- 

 ses from the foot-plates of the cone-cells and with the end-knobs of the rod-fibres. 

 In addition to these constituents of the plexiform layer, numerous fibres arising from 

 the protoplasmic processes of the horizontal cells of the inner nuclear layer also take 

 part in its formation. 



Internal limiting 

 membrane 



Ganglion cell 



Fibres of Miiller 



Bipolar nerve- 

 cells 



Blood-vessel 



Layer of visual 

 cells 



Nucleus of cone- 

 cell 



Pigment layer 

 Section of human retina from near posterior pole. X 230. 



