THE NERVOUS TUNIC. 



1467 



cells are present as an irregular layer within the fused remains of the two plexiform 

 layers. The most conspicuous elements are the visual cells, which in this position are 

 represented solely by the cones, which 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-called 



Internal limiting 



membrane Inner plexiform layer 



Fig. 1223. 





Ganglion cells 



Fovea centralis 



Vacuole 



Pigmented cells 



Inner cells 



Bipolar cells Outer Pigmented Cone Cones 

 plexiform layer layer visual cells 



Section of human retina through fovea centralis. X 80. 



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 irregu- 

 lar line, the ora serrata. Within a zone of about i mm. in width, the retina dimin- 

 ishes in thickness from .50 to .15 mm., in consequence of the abrupt disappearance 

 of its nervous elements. The rods disappear first ; then the cones become rudimen- 

 tary, and finally cease ; the ganglion cells, nerve-ribre 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 

 developed. These elements 



continue beyond the ora ^^' '^'^'^^• 



serrata (Fig. 1224) as the 

 transparent cylindrical cells 

 composing the inner layer of 

 the pai^s ciliaris retina; , the 

 densely pigmented cells of 

 the outer layer being a direct 

 continuation of the retinal 

 pigmented cells. These two 

 strata of cells are prolonged 

 over the ciliary body and the 

 iris as far as the pupillary margin, over the iris constituting the pars iridica retince. 

 As the columnar cells pass forward, they gradually decrease in height, and at the 

 junction of the ciliary body and the iris the cells of both layers become deeply pig- 

 mented, with consequent masking of the boundaries of the individual elements. 

 The cells of the anterior layer are of additional interest as giving rise to the dilatator 

 muscle of the iris. 



The aggregation incident to the convergence of the nerve-fibres from all parts of 

 the retina produces a marked thickening of the fibre-layer around the optic disc, 

 and as the fibres turn outward to form the optic nerve the other layers of the retina, 

 together with those of the choroid, suddenly cease. On the temporal side a 

 narrow meshwork of intermediate tissue separates the nerve-fibres from the other 

 retinal strata, but at the nasal side this tissue is absent. The ganglion cells dis- 

 appear first, whilst the pigmented cells^ with the lamina vitrea of the choroid, 

 extend furthest inward. 



The blood-vessels of the retina are derived from a single artery, the arteria 

 centralis retince, which enters the optic nerve at a point from 15-20 mm. behind 

 the eyeball, and, with its accompanying vein, runs in the axis of the nerve and 



Pigrmented 

 layer 



Section of human retina through ora serrata, showing transition of pars 

 optica into pars ciliaris. X 165 



