352 



only the absolutely essential retinal strata the pigment cells and the visual 

 cells with their necessary connections being retained within the area of 

 sharpest vision (Fig. 401). On approaching the fovea, the ganglion-cells 

 rapidly decrease in number, until, at the centre of the depression, they and 

 the nerve-fibre layer are entirely absent. The bipolar cells are present as an 

 irregular layer within the fused remains of the two plexiform layers. The 

 most conspicuous elements are the visual cells, in this position represented 

 solely by the cones, that have about twice their usual length and thickness, 

 the increase in length being contributed by the outer segments. The cone- 

 cell nuclei become removed from the external limiting membrane; the cone- 

 fibres are therefore lengthened, pursue a radial direction, and constitute the 

 so-ca[\ed. fibre-layer of Henle. Opposite the centre of the fovea, the choroid 

 is thickened by an increase in the choriocapillaris. The yellow color of the 

 macula is due to a diffuse coloration of the inner retinal layers. 



The Ora Serrata. The visual part of the retina ends anteriorly in 

 an irregular line, the ora serrata. The retina diminishes in thickness in con- 

 sequence of the abrupt disappearance of its nervous elements. The rods 

 disappear first; then the cones become rudimentary, and finally cease; the 

 ganglion-cells, nerve-fibre layer and inner plexiform layer fuse, and the two 

 nuclear layers unite and lose their characteristics, most of the nuclei present 

 being those of the supporting fibres of Miiller, which are here highly devel- 

 oped. These elements continue beyond the ora serrata (Fig. 402) as the 

 transparent cylindrical cells composing the inner layer of the pars ciliaris 



cells 

 Inner cells 



FIG. 402. Section of human retina through ora serrata. showing transition of pars optica into pars 



ciliaris. X 165. 



retina, the densely pigmented cells of the outer layer being a direct contin- 

 uation of the retinal pigmented cells. These two strata of cells are pro- 

 longed over the ciliary body and the iris as far as the pupil, over the iris 

 constituting the pars iridica retintz. As the columnar cells pass forwards, 

 they gradually decrease in height, and at the junction of the ciliary body 

 and the iris the cells of both layers become deeply pigmented. 



The blood-vessels of the retina are derived from the central artery, 

 which enters the optic nerve behind the eyeball, and, with its accompanying 

 vein, runs in the axis of the nerve until it emerges slightly to the nasal side 

 of the centre of the optic disk. Here the artery divides into two short supe- 

 rior and inferior branches, each of which subdivides into nasal and temporal 

 branches which give off end-arteries, no anastomosis existing. The macular 

 region is supplied by special twigs, the centre of the fovea, however, being 

 free from blood-vessels. The larger branches course within the nerve-fibre 

 layer, and send fine twigs to form an inner and an outer plexus, the former 

 on the outer surface of the inner plexiform layer, and the latter within the 

 inner nuclear layer. Beyond the outer plexiform layer the vessels do not 

 penetrate, the visual cells being dependent for their nourishment upon the 

 choriocapillaris of the choroid. The lymphatics of the retina are repre- 



