446 HISTOLOGY 



they terminate in relation with dendrites and cell bodies of large ganglion 

 cells which constitute the ganglion of the optic nerve. Cell bodies of this 

 ganglion form the ganglion cell layer, and their neuraxons, traveling toward 

 the papilla of the optic nerve, are the principal elements in the nerve fiber 

 layer. The latter is separated from the vitreous body by an internal 

 limiting membrane. Thus visual stimuli, received by the rods and cones, 

 are transferred by means of the bipolar cells of the ganglion retinas, to the 

 ganglion cells of the optic nerve, through the neuraxons of which they 

 proceed to the brain. These layers may be described in further detail as 

 follows : 



Henle's fiber layer contains not only the fiber-like basal ends of the 

 rod and cone cells, but also the slender unbranched dendritic processes 

 of the bipolar cells of the ganglion retinae. Each bipolar cell sends one 

 such process through Henle's layer to terminate in a little thickening 

 near the membrana limitans externa. In the outer reticular layer, how- 

 ever, these dendrites of the bipolar cells send out branches which bifurcate 

 repeatedly, becoming reduced to the finest fibrils; they form a close sub- 

 epithelial felt- work (Fig. 454). 



Occasionally nuclei are found in the outer reticular layer. Most of 

 these belong with bipolar cells displaced outward (Fig. 454, x). Toward 

 the inner nuclear layer, however, there are stellate ganglion cells with 

 neuraxons which pursue a horizontal course and then turn inward to join 

 the optic nerve fibers, as shown in Fig. 454. The existence of such fibers 

 has been denied by some writers. The neuraxons of other stellate gang- 

 lion cells in this region end in relation with the bases of the visual cells 

 (Fig. 454, +). 



Toward the inner reticular layer, the inner nuclear layer contains 

 the bodies of ganglion cells, which appear to lack a- chief or large process, 

 and are therefore called "amakrine" cells. They send branching fibers 

 into the inner reticular layer, where they interlace with the fine varicose 

 branches of the bipolar cells, and with the ramifications of the dendrites 

 from the ganglion nervi optici. 



The ganglion cell layer consists of a single row of large multipolar 

 cells containing Nissl's bodies. Certain of these cells because of excep- 

 tional size are known as "giant ganglion cells," and they occur at quite 

 regular intervals. "Twin cells" have been found, consisting of two cell 

 bodies united by a short bridge; only one of the pair has a neuraxon. 



The nerve fiber layer consists chiefly of the non-medullated neuraxons 

 of the ganglion cells, arranged in plexiform bundles. Occasionally the 

 neuraxons send collaterals back to the ganglion cell layer, where they 

 branch about the cell bodies (Fig. 454). The fiber layer contains also 

 neuraxons which have come out from the brain to terminate in free 

 branches among the cells of the inner nuclear layer. 



