VII 



RETINAL EXCITATION 



333 



Fie. 157. Outer surface of layer 

 of rods and cones. 350 dia- 

 meters. (Kollikcr.) ., part 

 of macula lutca, where there 

 are only cones ; 6, part near 

 the macula, with only one row 

 of rods between the cones ; 

 c, part of the retina midway 

 between the macula and the 

 ora serrate, where rods pre- 

 ponderate. 



The principal elements of the inner nuclear layer are small 



bipolar nerve-cells, owing to which this layer has also been termed 



the ganglion retinae. Some of these cells 



by their arborescences bring the termina- 

 tions of the rods into connection with the 



ganglion cells of the seventh layer : others 



come into contact with the ends of the 



cone-fibres in the outer molecular layer, 



and with the protoplasmic processes of the 



ganglion cells of the seventh layer, by means 



of their processes, which penetrate the inner 



molecular layer to different depths. 



Fig. 159 is a 

 diagrammatic 

 reconstruction 

 of the retina. In 

 addition to the 



elements of the rods and cones, the bipolar 

 cells and the optic ganglion cells (which 

 form a chain of neurones), certain others 

 can be seen, the morphological nature 

 and the real function of which are not 

 exactly known. 



At the limit of the outer molecular 

 layer are the so-called " horizontal cells " 

 ^ r-j of Eamon y Cajal, which vary in form 

 mV an d size, and are apparently intended to 

 Y bring a number of different rods into 



association. 



At the outer limit of the inner 

 nuclear layer there are other cells which 

 send all their processes into the inner 

 molecular layer (spongioblasts, W. Miiller ; 

 parareticular cells, Kallius). In some of 

 these cells no axis cylinder can be demon- 

 strated (amacrine cells, Cajal). Along 

 with these there are in some animals 

 cells with long processes, which pass 

 directly into the eighth layer, and run 

 with the fibres of the optic nerve. 



1 1... i58.-Diagram to show some of The optic nerve-fibres, which compose 



the neuro- epithelial elements of ... , ' . 



the retina. (From Schwaibe, the eighth layer, lose their medullated 



modified.) Numbered as in Fijr. i_ j.i_ 1 "M /"r> 



I,*,!, sheath on entering the papilla (Bow- 



mann), and spread as naked axons in all 



directions over the retina. The depth of this layer therefore 

 diminishes regularly from the papilla to the ora serrata. Most of 

 the fibres represent the nerve processes of the ganglion cells of the 



