STRUCTURE OF THE RETINA 



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The outer limb is a cylindrical rod about 

 30/4 long by 2^ in diameter. It is trans- 

 parent and composed of doubly refrac- 

 tive material. The inner limb of the 

 cell is about the same length as the outer, 

 is similar, and is longitudinally striated, 

 and contains a nucleus on its course, 

 figure 447, d. 



The cone cells are also made up of 

 two limbs, the outer of which is conical 

 instead of cylindrical as in the case of 

 the rods. In other respects they are 

 similar to the rods in structure, with the 

 exception that the inner limb ends in a 

 brush of fibrils which interlace with the 

 bipolar cells of the middle layer. In 

 man and mammals the number of rod 

 cells is much greater than the cones, 

 but it is said that in birds cones predom- 

 inate. Even in man the center of the 

 fovea centralis is devoid of rods and con- 

 sists of cones only, figure 450. 



All the elements of the retina are 

 sustained and isolated by large cells 

 lying vertically which are known as the 

 fibers of Muller. The nucleus of the 

 fiber of Muller is found at the level of 

 the layer of bipolar cells. The two ex- 

 tremities of the protoplasm or cell body 

 are condensed in two homogeneous 

 layers, known as the external and the 

 internal limiting layer. The external 

 limiting layer is placed just between the 

 two segments of the rod and cone cells, 

 forming a fenestrated membrane. The 

 internal limiting layer is situated upon 

 the internal surface of the retina. 



At the ora serrata the highly special- 

 ized structure of the retina disappears. 

 The nerve fibers and ganglion cells dis- 

 appear, the connecting cells are fewer, 

 the cones more sparse, and the rods 

 shorten and disappear. The structure 



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FIG. 450. Diagrammatic Section of 

 the Macula Lutea and Fovea Centralis, 

 2, Layer of nerve fibers; 3, layer of 

 multipolar cells; 4, internal molecular 

 layer, composed of intertwining arbor- 

 escent processes; 5, layer of bipolar 

 cells, or internal granular layer; 6, 

 external molecular layer, composed 

 of intertwining arborescent processes; 

 7, nuclei of epithelial cells, or external 

 granular layer; 8, frillwork formed 

 by processes from fibers of Muller, 

 often called the "external limiting 

 membrane"; 9, layer of rods and cones; 

 10, layer of pigment epithelium. 



