THE RETINA, OR TUNICA INTERNA 



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arborizations around the ends of the rod fibres and with the branched foot plates of the cone 

 fibres. In the neighborhood of the macula lutea, elongations from the inner segments of rod 

 fibres and cone fibres form the so-called Henle's fibre layer. 



7. The outer nuclear or outer granular layer, like the inner nuclear layer, contains several 

 strata of clear oval nuclear bodies; they are of two kinds, and on account of their being respec- 

 tively connected with the rods and cones of Jacob's membrane (rod fibres and cone fibres) are 

 named rod granules and cone granules. The rod granules are much the more numerous, and 

 are placed at different levels throughout the layer. Their nuclei present a peculiar cross-striped 

 appearance, and prolonged from either extremity of the granule is a fine process; the outer 

 process is continuous with a single 

 rod of Jacob's membrane; the inner 

 passes inward toward the outer 

 molecular layer and terminates in an 

 enlarged extremity, and is embedded 

 in the tuft into which the outer pro- 

 cess of the rod bipolars break up. 

 In its course it presents numerous 

 varicosities. The cone granules, 

 fewer in number than the rod gran- 

 ules, are placed close to the mem- 

 brana limitans externa, through 

 which they are continuous with the 

 cones of Jacob's membrane. They 

 do not. present any cross-striping, 

 but contain a pyriform nucleus 

 which almost completely fills the 

 cell. From their inner extremity a 

 thick process passes inward to the 

 outer plexiform layer, upon which 

 it rests by a somewhat pyramidal 

 enlargement, from which are given 



External 

 segment 



Intermediary 



disc 

 Elliptoid- 



My old 

 Nucleus 



FIG. 813. The cells of the rods of the 

 retina in the frog. A. Red rod. B. Green 

 rod. (Poirier and Charpy.) 



FIG. 814. Cones in the different regions of the retina. 7. Near 

 the ora serrata. 77. At 3 mm. from the ora serrata. 777. At an 

 equal distance from the ora, serrata and the papilla. IV. At the 

 periphery of the fovea centralis. V. In the fovea centralis. VI. 

 At the centre of the fovea central^. E. Length of the external 

 segment. /. Length of the internal segment. D. Diameter of 

 the internal segment. (Poirier and Charpy.) 



off numerous fine fibrils, which enter the outer plexiform layer, where they come in contact 

 with the outer processes of the cone-bipolars. 



8. The membrana limitans externa, like the membrana limitans interna, is derived from 

 the fibres of Miiller, with which structures it will be described. 



9. Jacob's membrane, or the layer of rods and cones, consists of visual cells, and the ele- 

 ments which compose it are of two kinds, rod cells and cone cells, the former being much more 

 numerous than the latter. The rod cells (Fig. 813) are of nearly uniform size, and arranged 

 perpendicularly to the surface. A rod cell consists of a rod and a rod fibre, and the fibre con- 

 tains the nucleus. The rods are cylindrical and each consists of two portions, an outer segment 

 and an inner segment, which are of about equal length. The segments differ from each other 

 as regards refraction and in their behavior with coloring reagents, the inner portion becoming 



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