BIPOLAR AND GANGLION LAYERS 47 



between visual cells and bipolars. This is the 'outer plexiform layer'. 

 An 'inner plexiform layer' also occurs on the vitread side of the inner 

 nuclear layer, and has a similar significance. In it lie the synaptic junc- 

 tions between bipolar cells and the innermost of the three masses of cells 

 concerned with the projection of the image to the brain — the 'ganglion 

 layer'. 



The Ganglion Layer — The cells of this layer (Fig. 19) have either 

 small or large bodies and simple or elaborate dendrites which reach up 

 into the inner plexiform layer to meet the termini of the bipolars. Each 

 ganglion cell gives off a slender axon process which courses along the 

 inner surface of the retina, next to the vitreous. All of these fibers, from 



Fig. 21 — The optic chiasma. 



a, of man, showing partial decussation of optic nerve fibers. 



b, of bird, showing total decussation; in some vertebrates (i.e., most fishes) the nerves are 

 not thus plaited — but whether the fibers are interwoven or not, they all decussate in non- 



mammals. 



c- chiasma; n- optic nerve; 



retina; /- optic tract, which enters brain. 



all over the sensory retina, converge at one place in the 'fundus' (back) 

 of the eye and there turn parallel to each other and pass outward 

 through the retina, chorioid and sclera in a compact bundle as the optic 

 nerve, which travels toward the brain (Fig. 21). 



A ganglion cell may gather in the axons of several bipolars (Fig. 19, 

 pg) , just as one of the latter in turn often connects not with one visual 

 cell but with several. This has been called the 'inward convergence' of 

 the visual cells upon optic nerve fibers, or 'summation'. The impulses 

 which travel down several visual-cell foot-pieces are summated in their 

 efforts to stimulate a single bipolar cell, and numbers of bipolar nerve- 

 impulses are in turn gathered into single ganglion cells and optic nerve 



