LAYERS OF THE HKTTXA. 45 



same way as do the cells of the ganglion-cell layer, that is to say, partly in the 

 varying size of the cell-body, and the character of the cell-processes, partly in the 

 position of their terminal arboresccnces within the inner molecular layer. 



The varieties of amacrine cells described by Cajal are firstly, those with diffusely ramified 



their arbore.scence extending? throughout the whole depth of the molecular layer 



(tig. .10. !..//; vi., D). and secondly, those having their terminal arborescences horizontally 



placed in the several strata of the molecular layer. Of these so-called stratified amacrines 



there niv. ,-i> in the case of the ganglion-cells, three principal types 



(1.) Those of the first type (fig. 50, v., B, E ; vi., B) have for the most part very large 

 cell-bodies and a thick stalk-like process, sometimes more than one, extending into the inner 

 molecular layer, and ramifying in one or other of its strata over a considerable extent of area, 

 but with comparatively few and relatively coarse processes. A good deal of variation is, 

 however, met with in the character and extent of these processes. 



(2.) Those of the second type (Jig. .10, v., F, G, H ; vi., E, F, G ; vn., B, C) have a pyriform 

 cell-body of medium size with a straight stalk passing into the molecular layer, and ending in 

 one of its strata in a moderately extended close interlacement of fibrils. 



(8.) Th' 1 amacrines of the third type (fig. 50, v.. D ; vi., C) are of small or medium size, 

 usually with a fine stalk-like process passing into the molecular layer. From the lower 

 inner) end of this process a terminal tuft of very fine radiating fibrils spreads out in one of 

 the strata of the molecular layer, the extent of the arborescence thus formed being often very 

 considerable. When, however, the arborescence ife near the inner nuclear layer, the fibrils 

 may come off from the body of the cell (fig. 50, v., vi., A), 



r. X/)o /tf/t n h lasts of outer molecular layer : horizontal cells of Cajal : basal cells. 

 These are flattened or irregularly projecting cells, the bodies of which occupy the 

 outermost part of the inner nuclear layer, whilst their greatly ramified processes 

 i \tctnl into and end in the outer molecular layer. The stratum in which they lie is 

 termed by Cajal the layer of horizontal cells; it was previously described by 

 \V. Krause as the membrana fenestrata. 



T\vo kinds of these cells have been noticed by Cajal in mammals, and by their 

 situation they serve to subdivide the layer into two strata, an inner and an outer. 



(1.) The cells in the inner stratum of the layer are large and broadly pyramidal in shape. 

 the base being directed towards the outer molecular layer, and resolving itself into a large 

 number of coarse but rapidly tapering processes which end in small tufts of thort varicose 

 vertical fibrils at about the level in the outer molecular layer in which the knobs of the rod- 

 fibres occur. The apex of the pyramid is sometimes truncated, but in other cases can be 

 traced as a thick vertical process down into the inner molecular layer, where it ends in 

 horizontal branches. Each cell has a long axis-cylinder process, which extends for a con- 

 siderable distance within the outer molecular layer to end in a closely interlaced terminal 

 ramification (fig. 50, II., B, C ; IV., a). 



(2.) Semilunar cells, from the upper (outer) flattened surface of which a thick brush of 

 closely interlacing radiating filaments comes off and passes vertically outwards towards the 

 base of the rod- and cone-fibres (fig. 50, II., A). These also have an axis-cylinder process 

 which passes horizontally and a little upwards to end in the outer part of the outer molecular 

 layer. 



5. Outer molecular layer. The outer molecular layer is much thinner than 

 the inner, but otherwise presents, in vertical sections of hardened retina, a similar 

 granular appearance. 



This layer (fig. 50, n.) is largely formed of the processes of the horizontal cells 

 which have just been described, and also of the outwardly directed protoplasmic 

 processes of the bipolars, which ramify within it and interlace with similarly 

 ramifying fibres from the cones, and with the knobbed ends of the rod-fibres. 



In some mammals there are cells with widely extending branched processes 

 resting upon the outer surface of the outer molecular layer (fig. 50, in., h), much in 

 the same way as some of the amacrine cells rest upon the inner molecular. The 

 outer molecular layer also receives fine axis-cylinder processes which pass into it from 

 the inner molecular (fig. 50, r., /'), but whence they are derived is not known. 



