636 



THE SENSES 



ganglionic cells, and one process externally. This process is often divided 

 into many branches, which separate out into a horizontal brush, interlacing 

 with the processes of the rods and cones. Special cells have been described 

 for this layer of the retina, as, for example, the spongioblasts of Cajal. 



The External Layer of Rods and Cones. The rod cells are composed of 

 two parts quite different in structure, known as the outer and inner limbs. 

 The outer limb is a cylindrical rod about 30 /j. long by 2 p. 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 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. 



FIG. 449. Distribution of the Rods and Cones. A, In the peripheral part of the retina; 

 B, from the region of the macula lutea. 



In man and mammals the number of rod cells are much greater than the cones, 

 but it is said that in birds cones predominate. 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 Mutter. The nucleus of the 

 fiber of Miiller is found at the level of the internal granular layer, and the 

 two extremities of the protoplasm or cell body are condensed in two homo- 

 geneous 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. The internal limiting layer is situated upon the internal 

 surface of the retina. 



At the ora serrata the layers are not perfect and disappear in this order: 

 nerve fibers and ganglion cells, then the rods, leaving only the inner limbs of 

 the cones, these cease, then the inner molecular layer. The Mlillerian fibers 

 persist. 



