THE NERVOUS TUNIC. 



1465 



Fig. 1221. 



The inner nuclear layer, the most complicated of the retinal strata, measures 

 .035 mm. in thickness near the optic disc. It contains nervous elements of three 

 main types — the horizontal cells, the bipolar cells, and the amacrine cells — and, 

 associated with these, the nuclei of the sustcntacular cells. 



The horizontal cells form the external layer, and were formerly included in the 

 outer plexiform layer. They have flattened cell-bodies and send out from five to 

 seven dendrites, which divide into innumerable branches and, passing into the outer 

 plexiform layer, terminate in close association with the bases of the rod and cone 

 visual cells. Each horizontal cell possesses also an axone, which is directed outward 

 through the outer plexiform layer, and ends in a richly branched arborization about 

 the visual cells. A second type of large horizontal cells 

 is also described, some of which send axis-cylinder pro- 

 cesses through the inner nuclear layer to form terminal 

 arborizations in the inner plexiform layer. The function of 

 the horizontal cells is not well understood, but they prob- 

 ably serve as association fibres between the visual cells. 



The bipolar cells, the ganglion cells of this layer, 

 are of two chief varieties, the rod-bipolars and the cone- 

 bipolars. They are oval cells, each sending an axone 

 inward toward the inner plexiform layer, which ends in 

 communication with the large nerve-cells of the ganglion 

 cell layer, and a dendrite outward which is associated with 

 the end terminals of the visual cells and with the arboriza- 

 tions of the horizontal cells. The dendrites of the rod- 

 bipolars form an arborescence of vertical fibrils, which 

 enclose from three to twenty end knobs of the rod-fibres, 

 whilst their axis-cylinders pass entirely through the inner 

 plexiform layer and usually embrace the cell-body of one 

 of the large ganglion cells. The dendrites of the cone- 

 bipolars, on the other hand, bear horizontal arborizations 

 which interlace with the fibrils from the foot-plates of the 

 cone-cells. Their axones penetrate less deeply into the 

 inner plexiform layer than do those of the rod-bipolars, 

 coming in contact at various levels with the peripherally 

 directed dendrites of the ganglion cells. 



The amacrine cells are placed in the inner portion of 

 the nuclear layer. Formerly considered as sustcntacu- 

 lar elements, they are now recognized as nerve-cells, 

 although, as their name indicates, no distinct axone can 

 be demonstrated. They possess, however, richly branched 

 dendritic processes, which ramify in the inner plexiform 

 layer and end either as the brush-like arborizations of the 

 diffuse amacrines, or as the horizontally branching arborizations of the stratiform 

 atnacrines. A third type, known as association amacrines, is also described. They 

 connect widely separated amacrine cells of the same layer (Cajal). 



The nuclei of the sustcntacular cells, the fibres of Miiller, will be described later 

 (page 1466). 



The inner plexiform layer, .04 mm. in thickness, appears granular, similar 

 to the corresponding outer zone, and is composed of the interlacing axones of the 

 bipolar, amacrine and horizontal cells from the inner nuclear layer and the dendrites 

 of the large ganglion cells in the subjacent retinal layer. Intermingled with them 

 are also the fibres of Miiller, which form conspicuous vertical striae, with lateral 

 offshoots within the stratum. 



The layer of ganglion cells, consists, throughout the greater part of the 

 retina, of a single row of large multipolar neurones, each with a cell-body containing a 

 vesicular nucleus and nucleolus and showing, like many other ganglion cells of the 

 central nervous system, typical Nissl bodies and a fibrillar structure. Neat the 

 macular region, the ganglion cells are smaller but more numerous and arranged as 

 several superimposed layers; toward the ora serrata, on the contrary, the individual 



Visual cells from human ret- 

 ina, A, cone-cell; B, rod-cell; a, 

 b, outer and inner segments; c, 

 attenuated bodies (fibres), with 

 nucleus (d) and central ends (e)\ 

 em, position of external limiting 

 membrane. X 750. {Greeff.) 



