44 



THE EYE. 



the cone-bipolars have their horizontal arboresceiice extending over a large area of 

 the outer molecular layer (fig. .")(), i.,/, in.,/, y) 9 and probably come into contact with a 

 considerable number of cone-feet. 



In birds, reptiles, and amphibia some of the bipolar cells probably correspond- 

 ing to those described above as cone-bipolars give oft' from their arboresceiice 

 within the outer molecular layer an uubranched irregularly varicose fibril as far as 

 and just beyond the meinbrana limitans externa (fibril of Land oil,}, where it usually 

 ends in a clavate enlargement (fig. 51, E). According to Cajal it is absent in 

 mammals and teleosteans. 



The relative length of the inner and outer process of the bipolars naturally 

 differs according to the position of the individual cell in the nuclear layer : if the 

 cell is near the inner molecular layer the outer process will have a longer course to 

 reach the outer molecular layer, and, conversely, if the cell is near the latter. At 

 and near the central fovea these processes or fibres of the inner nuclear layer have a 



Fig. 51. SECTION OF 



KKTJXA, PREPARE!) BY 

 GOLGl's METHOD. (11. V 



Cajal.) 



A, i), Inrge spongioblasts 

 of inner nuclear layer ; C, 

 smaller spongioblast of the 

 same layer ; D, small bi- 

 polar cells with one process, 

 a, b, ending in terminal rami- 

 fications in the inner molecular 

 layer, and the other process 

 ending partly in a flattened 

 ramification in the outer mole- 

 cular layer and partly in a 

 filament which ends at the 

 external limiting membrane 

 in an enlarged extremity 

 (at E) ; F, Gr, rod and cone 

 nuclei ; H, I, cells with 

 ramifications in the outer 

 molecular layer ; J, fibre of 

 M tiller. 



markedly oblique direction, in other parts of the retina they run nearly vertically to 

 the surfaces. 



In the frotf and lizard it is common to find bipolars amongst the outer granules as well as 

 in their ordinary position in the inner nuclear layer. Such cells are spoken of as displaced 

 bipolars. 



b. HponyioWasts (of inner molecular layer) of "VV. Miiller : amacrine * cells of 

 Cajal. These, which are placed in the inner part of the inner nuclear layer, form 

 an almost complete stratum, which is termed by Cajal the layer of amacrine cells. 

 As this name implies, it has not hitherto proved possible to demonstrate the 

 existence of an axis-cylinder process in these cells (which are nevertheless regarded 

 by Cajal as nerve-cells 2 ), but they possess, on the other hand, extensively ramified 

 protoplasmic processes which are wholly included within the inner molecular layer, 

 and mostly form horizontal arborisations in the several strata of that layer. 

 There are several kinds of these cells, which differ amongst one another much in the 



' a, privative ; /istKpds, long ; tt>os, fibre. 



2 In birds, reptiles, and amphibia there are a certain number of large cells in this layer which have 

 an undoubted axis-cylinder process extending into the nerve-fibre layer (Dogiel) (see fig. 51, A), but 

 they have not been found in mammals. 



