2 5 6 



HISTOLOGY 



indicated by the special processes of staining (Fig. 225). The inner- 

 most layer, which is a distal region when explained embryologically, 

 is a layer of nerve fibers passing in many directions 

 and all terminating at the blind spot, where they 

 dip down through all the other layers to become 

 the optic nerve. Just under this layer is found a 

 scattered layer of ganglion cells, which form the 

 ganglion-cell layer. This last is followed by a thicker 

 fibrous layer termed the inner nuclear layer. This 

 is composed of many small, round nuclei about five 

 deep. On its outer margin comes the thin inner retic- 

 ular layer, and this is marked off from the thicker 

 Henle's fiber layer by an incomplete membrane. 



Next to Henle's layer is found the outer nuclear 

 layer, which is like the inner except that it is thicker, 

 being about six or seven nuclei deep. Its outer 

 side is marked by the sharp and clear outer lim- 

 iting membrane. From this membrane the rods 

 and cones of the eye project distally as a thicker 

 layer. They are both of equal height, and each is 

 composed of two segments. The rod segments are 

 about equal in length and show no great differen- 

 tiation. With the cone it is different, for the 

 basal segment is of huge cone-shaped bulk, many 

 times as thick as the rods ; while the distal segment 

 is very much like that of the rod. 



The rods and cones are particularly well seen 

 in some fish, as in the herring. Figure 226 repre- 



FIG. 226. Part of the sents a cone an( ^ severa l r ds from this fish. The 

 retina of a herring, show- cone is not as high proportionally as in man, and 



; n rg ans e l7w k hrctth f e C vt P"? of the ^ *^* has been cut off in pre- 

 uai cells perceive light, paring the section. This preparation is particu- 

 Ten rods (r&.) and one j^ j happy in showing the transverse lamellae of 



cone (c.). nu., nuclei / \rJ 6 



of the rod cells. TWO which the rods are composed. The rods and 



nuclei visible in the cone cones f orm the distal layer Q f this waU Q f the 

 cell, x 1000. .... 



ongmal optic cup, or eye-sac. The opposite wall, 

 however, has been closed in and intimately applied to them, so that it 

 appears to be and is indeed another layer of the retina. It is a simple 

 layer of cuboidal cells, filled with pigment in which the distal ends of the 

 rods are buried. When the light becomes too bright, the cytoplasm of 

 these cells is pushed up between the rods, and thus it partially shades 

 them. This is a very different condition from that found in many other 

 eyes. 



