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 Miiller, 

 often called the "external limiting 

 membrane"; 9, layer of rods and cones; 

 to, layer of pigment epithelium. 



SENSES 



The outer limb is a cylindrical rod 

 about 30^ long by 2/1 in diameter. It 

 is transparent and composed of 

 doubly refractive 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 re- 

 spects 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 pre- 

 dominate. Even in man the center of 

 the fovea centralis is devoid of rods 

 and consists 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 Miiller. The nucleus 

 of the fiber of Miiller is found at the 

 level of the layer of bipolar cells. 

 The two extremities of the proto- 

 plasm 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 

 specialized structure of the retina 

 disappears. The nerve fibers and 

 ganglion cells disappear, the con- 

 necting cells are fewer, the cones 

 more sparse, and the rods shorten 



