670 



THE SENSES 



The Nerve Fiber and Nerve Cell Layers. The inside of the retina i 

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

 nerve cells and run toward the exit of the optic nerve. Externally the gan 

 glionic cells send up numerous processes, or dendrites, which interlace witl 

 the fibers of the bipolar cells of the second layer. 



FIG. 448. Perpendicular Section of the Retina of a Mammal. A, External grains o 

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

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

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

 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 sen( 

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

 ganglionic cells, and one process externally. This process is often divide< 

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



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 describee 

 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 o 

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



