INTERCONNECTION OF RETINAL ELEMENTS. 



57 



definitely to trace the course and connections of the fibres which pass from and to 

 the several retinal elements. The most fruitful applications of Golgi's method have 

 been made by Ramon y Cajal, following up investigations by Dogiel, which 

 were made by Ehrlich's method ; it is the account given by the first-named 

 observer which will here in the main be followed. 



In the first place, it would appear that there is no direct anatomical continuity 

 between the elements of the several layers, with the exception of some of the 

 nerve-fibres of the first layer and the ganglionic cells immediately outside them. 

 As with other parts of the nervous system (see Part I. of this volume) the 

 nerve-elements of the retina are anatomically isolated units, merely coming into 

 connection with one another by the interlacement of their arborescent processes. 

 These interlacements occur in two places, viz., in the two molecular layers. In the 



Fig. 65. DIAGRAM SHOWING THE MODE OP CONCATENATION OP THK 



VISUAL NERVOUS ELEMENTS IN THE VERTEBRATE TYPE. 



outer molecular layer is found the interlacement by 

 which the rod- and cone-elements are brought into 

 connection with the inner granules. In the inner 

 molecular layer there is a series of interlacements 

 running mainly in planes parallel to the surfaces of the 

 layer, and serving to bring the elements of the inner 

 nuclear layers into connection with those of the gang- 

 lionic layer. Finally, some of the nerve-fibres of the 

 first or innermost layer ramify directly in the mole- 

 cular layer or pass through this layer and ramify 

 amongst the inner granules (figs. 50, 51, 64). 



The retina, therefore, is essentially formed by a 

 number of nerve-cell chains, the elements of which are 

 arranged in three series from without in. The first of 

 these is formed by the rod- or cone-element. One 

 end of this element abuts against or is imbedded 

 in a pigment-cell, the other end interlaces by the 

 terminal arborisation of the rod- or cone-fibre within 

 the outer molecular layer, with the peripheral arborisa- 

 tions of the next elements. The latter are the bipolar inner granules (fig. 65, gr.i). 

 These, by the peripheral process just mentioned, interlock with the arborisations 

 of the rod- and cone-fibres and in some animals also send the fibres of Landolt 

 as far as the membrana limitans externa. By their central processes they ramify 

 within the inner molecular layer (m.i.) and interlace with the peripheral processes 

 of the ganglion-cells (g). The last-named form the third of the concatenated 

 elements. Their peripheral processes spread out in the inner molecular layer, and 

 are connected with the central processes of the inner granules in the manner just 

 stated. Their central process (n) is an axis-cylinder of one of the fibres of the 

 optic nerve, and its terminal ramification is to be found in the grey matter of the 

 superior corpora quadrigemina, or of the lateral geniculate bodies. The functions 

 of the other cell elements in the retina, such as the horizontal cells, which ramify 

 in the outer molecular layer, and the amacrine-cells of Cajal, which are distributed 

 in the inner molecular layer, are still quite obscure. 



