720 



THE SENSES 



optic nerve is said to be upward of 500,000 in each nerve (Salzer). The 

 fibers of the optic nerve spread out over the inner surface of the retina as 

 far as the ora serrata. 



The retina itself consists of layers of nerve elements supported by deli- 

 cate connective tissue. The older descriptions recognize some eight or ten 

 layers in the retina. These appear in the ordinary microscopic prepara- 

 tions and are shown in figure 446. But the newer investigations of Cajal, 

 Golgi, Retzius, and others have shown that the retina is a much simpler 

 structure than heretofore described. The retina is formed of essentially 

 three layers of nerve and sense cells. Naming from the center of the eye 

 outward, they are: The ganglionic layer; the layer of bipolar cells; and the 

 layer of rods and cones, figure 447. The cells of these layers have numer- 

 ous fibrous processes which interlock in such a way that they seem to form 

 different areas when studied in cross-section, figure 446. If we recognize 

 the strata of interlacing fibers, then the following may be made out : 



The layer of ganglion cells . . . 

 The layer of bipolar cells .... 



The layer of visual cells 



1. Ganglionic layer, with the fibers of the 

 optic nerve. 



2. Internal molecular layer. 



3. Internal nuclear layer. 



4. The external molecular layer. 



5. The external nuclear layer. 



6. The layer of rods and cones. 







FIG. 447. Transverse Section of a Mammalian Retina. A, Layer of rods and cones; 

 B, bodies of visual cells (external granular); C, external molecular layer; E, layer of bipolar 

 cells (internal granular); F t internal molecular layer; G, layer of ganglionic cells; H, layer 

 of optic-nerve fibers; a, rod; b, cone; c, body of the cone cell; d, body of the rod cell; e, 

 bipolar rod cells;/, bipolar cone cells; g, h, i, j, k, ganglionic cells ramifying in the various 

 strata of the internal molecular zone; r, inferior arborization of the bipolar rod cells, con- 

 necting with the ganglionic cells; r, inferior arborization of the bipolar cone cells; t, epithelial 

 or Miiller cells; x, point of contact between the rods and their bipolar cells; z, point of con- 

 tact between the cones and their bipolar cells; s, centrifugal nerve fiber. (Cajal.) 



