STRUCTURE OF THE RETINA 



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The Nerve Fiber and Nerve Cell Layers. The inner surface of the retina 

 is formed of a layer of nerve fibers which have their origin in the adjacent 

 large nerve cells and converge toward the exit of the optic nerve. Extern- 

 ally the ganglionic cells send up numerous processes, or dendrites, which 

 interlace with the fibers of the bipolar cells of the third or inner nuclear 

 layer and form the inner molecular layer of the older writers. 



FIG. 448. Perpendicular Section cu tne Retina of a Mammal. A t External grains or 

 bodies of rods; B, bodies of cones; a, horizontal external or small cell; b, horizontal internal 

 or large cell; c, horizontal internal cell with descending protoplasmic appendages; e, 

 flattened arborization of one of the large cells;/, g, h, j, l > spongioblasts ramifying in the 

 various strata of the internal molecular zone; m, n, diffuse spongioblasts; o, ganglionic cell; 

 i, external molecular zone; 2, internal molecular zone. (Cajal.) 



The Middle Layer. The middle layer consists of bipolar cells which send 

 one process toward the ganglionic layer to interlace with the dendrites of the 

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

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



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

 B, from the region of the macula lutea. 



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. 



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