1104 



THE ORGANS OF SPECIAL SENSE 



surface of each cell resting on the preceding layer and sending off an axone which is prolonged 

 as a nerve fibre into the fibre layer. From the opposite extremity numerous dendrites extend 

 into the inner plexiform layer, where they branch out into flattened arborizations at different 

 levels (Fig. 812). The ganglion cells vary much in size, and the dendrites of the smaller ones, 

 as a rule, Arborize in the inner plexiform layer as soon as they enter it; while the processes of the 

 larger cells ramify close to the inner nuclear layer. 



4. The inner plexiform layer is made up of a dense reticulum of minute fibrils, formed by the 

 interlacement of the dendrites of the ganglion cells with those of the cells contained in the next 

 la;er, immediately to be described. Within the reticulum formed by these fibrils, a few 

 branched spongioblasts are sometimes embedded. 



Membrana 



limitans interna"'( J. 



Nerve-fibre layer j_S 



Ganglionic layer- 



Inner plexiform 

 > layer 



Centrifugal fibre 



Inner nuclear... 

 layer 



Fibre of Mutter. 



Outer plexiform...^ 

 layer 



Outer nuclear .. 

 layer 



Membrana 

 limitans externa 



Layer of rods .. 

 and cones 



...Diffuse amacrine 

 cell 



'^Amacrine cells 

 'Horizontal cell 



t ?Eod bipolars 

 .-.-J*-. Cone bipolars 



o TO : '\~Pi9 me nted layer 



FIG. 812. Plan of retinal neurones. (After Cajal.) 



5. The inner nuclear or inner granular layer consists of a number of closely packed cells, of 

 which there are three varieties viz.: (1) A large number of oval cells, which are commonly 

 regarded as bipolar nerve cells, and are much more numerous than either of the other kind. 

 They each consist of a large oval body placed vertically to the surface, and containing a distinct 

 nucleus. The protoplasm is prolonged into two processes; one of these passes inward into the 

 inner plexiform layer, is varicose in appearance, and ends in a terminal ramification, which is 

 often in close proximity to the ganglion cells. The outer process passes outward into the outer 

 plexiform layer, and there breaks up into a number of branches. There are two varieties of these 

 bipolar cells one in which the outer process arborizes around the knobbed ends of the rod 

 fibres, and the inner around the cells of the ganglionic layer; these are called rod bipolars (Fig. 

 812); the others are those in which the outer process breaks up in a horizontal ramification, 

 in contact with the end of a cone fibre; these are the cone bipolars, and their inner process 

 breaks up into its terminal ramification in the inner molecular layer. (2) At the innermost 

 part of this inner nuclear layer is a stratum of cells, which are named amacrine cells (of Cajal), 

 from the fact that they have no axis-cylinder process, but they give a number of short proto- 

 plasmic processes which extend into the inner plexiform layer and there ramify (Fig. 812). 

 There are also at the outermost part of this layer some cells, the processes of which extend into 

 and ramify in the outer molecular layer. These are the horizontal cells (of Cajal). (3) Some 

 few cells are also found in this layer, connected with the fibres of Muller, and will be described 

 with those structures. 



6. The outer plexiform layer is much thinner than the inner plexiform layer; but, like it, 

 consists of a dense network of minute fibrils, derived from the processes of the horizontal cells 

 of the preceding layer and the outer processes of the bipolar cells, which ramify in it, forming 



