268 THE ESSENTIALS OF HISTOLOGY. 



trance it forms a slight eminence (colliculus nervi optici). The nerve- 

 fibres lose their medullary sheath on reaching the retina. Some of 

 the fibres pass through the ganglionic and molecular layers to form a 

 terminal arborisation in the inner nuclear layer (fig. 299). The layer 

 of nerve-fibres becomes gradually thinner in the anterior part of the 

 retina. 



The layer of nerve-cells, or ganglionic layer, is composed of nerve-cells 

 somewhat like the cells of Purkinje of the cerebellum but varying in 

 size, although those of large size are prevalent in most parts of the 

 retina. On the other hand, in the yellow spot small bipolar nerve-cells 

 are met with, and they may here lie several deep. These nerve-cells 

 have on the one side a fine axis-cylinder process prolonged into a fibre 

 of the layer just noticed, and on the other a thick branching process, the 

 ramifications of which terminate in the next layer in flattened arborisa- 

 tions at different levels (fig. 300, A, B, C). 



The inner molecular layer is comparatively thick, and has an appear- 

 ance very like the grey matter of the nerve-centres. A few nuclei are 

 scattered through it, and it is traversed by the processes of the nerve- 

 cells and of the inner granules, and by fibres from the optic nerve layer 

 as well as by the fibres of Miiller. 



The inner nuclear layer is mainly composed of bipolar nerve-cells con- 

 taining large nuclei (inner granules). The processes of these cells (fig. 

 301, D) extend on the one hand inwards into the inner molecular layer 

 where they spread out into terminal arborisations at different levels, 

 whilst the other process is directed outwards, and, after forming an 

 arborisation in the outer molecular layer, is continued on as far as the 

 external limiting membrane, where it appears to end in a free pointed 

 extremity (E). Besides these bipolar nerve-cells, there are other inner 

 granules which are different in character, having ramified processes 

 which only extend into one or other of the molecular layers, in which 

 the bodies of these cells are often partly embedded. The cells in ques- 

 tion are partly of the nature of neuroglia-cells (fig. 301, C and H), but 

 others (A, B, I) may perhaps be regarded as nerve-cells, since they have 

 been noticed to give off, besides branching processes or dendrites, which 

 ramify in the molecular layer, an axis-cylinder process which may 

 extend into the nerve-fibre layer. The fibres of Miiller have nucleated 

 enlargements (J) in the inner nuclear layer. 



The outer molecular layer is thin, and is composed mainly of the arborisa- 

 tions of the inner granules and of the rod-and-cone fibres (fig. 301, 5). 



As far as the outer molecular layer the retina may be said to consist 

 of nervous elements, but beyond this layer it is formed of modified 

 epithelium-cells. 



