64 THE LAYER OF GANGLIONIC CELLS. [BOOK in. 



fibrils in the outer molecular layer, and on the inner side a single 

 process which has very much the appearance of an axis cylinder 

 process (Fig. 145, x.) but which cannot be traced beyond the inner 

 molecular layer, though it has been supposed to pass on and 

 become an optic fibre. 



Thus while the nuclei of the outer nuclear layer are nuclei of 

 the rod fibres and cone fibres, the nuclei of the inner nuclear layer 

 are the nuclei -of cells of a peculiar character; most of them 

 belong to cells called bipolar, some of them, the innermost, belong 

 to so-called unipolar cells, while some of the outermost are de- 

 scribed as belonging to a third kind of cell. In several respects 

 this inner nuclear layer resembles the nuclear layer of the 

 superficial grey matter of the cerebellum. 



742. The layer of ganglionic cells is closely connected with 

 that of the optic fibres. The latter (Fig. 145, op. /.) consists, as 

 we have said, of nerve fibres, that is of naked axis cylinders 

 without medulla and without neurilemma, radiating in all 

 directions from the optic disc. Over the retina generally these 

 fibres exist as a single layer of bundles, arranged in a plexiform 

 manner, winding between the feet of the fibres of Miiller, and 

 supported by a neuroglia to which the processes of the fibres 

 of Miiller contribute, as well as by a scanty amount of connective- 

 tissue belonging to the retinal blood vessels. The fibres and 

 bundles of fibres become less numerous, and the plexuses more 

 open, from the optic disc towards the ora serrata, and at the 

 latter line cease altogether. The diminution is due to the optic 

 fibres continually passing away from the layer into the other, outer 

 layers. 



The layer of ganglionic cells is over the retina generally a 

 single layer of nerve cells. Each cell (Fig. 145, gc.\ 30 p or so 

 in diameter, consists of a cell body which is transparent or nearly 

 transparent, though sometimes containing pigment, and a rela- 

 tively large spherical nucleus. The cell-body sends inwards a 

 single undivided axis cylinder process which becomes an optic 

 fibre, and outwards a number of branched processes, which, 

 passing into the inner molecular layer and dividing, chiefly in 

 the middle zone of the layer, into delicate filaments, are lost to 

 view ; owing to these branched processes the cells are spoken of as 

 multipolar. These ganglionic cells are far less numerous than 

 the optic fibres, so that only some of the latter are continued 

 into the former; the rest of the fibres are either connected as 

 suggested with some of the cells of the inner nuclear layer or 

 possibly end without the intervention of cells, by branching in the 

 inner molecular layer in a manner similar to that of the processes 

 of the nerve cells. 



743. We may then consider the retina as made up of three 

 main layers of cells ; (1) the rods and cones with the outer nuclear 

 layer, (2) the cells forming the inner nuclear layer, and (3) the 



