VISION 



1015 



papilla) the optic nerve spreads over the retina as a layer of non- 

 medullated fibres, separated from the interior of the eyeball only 

 by the internal limiting membrane. This so-called membrane is formed 

 by the expanded feet of the fibres of Muller, which run like a scaffolding 

 or framework through nearly the whole thickness of the retina, ter- 

 minating at the outer limiting membrane. External to the layer of 

 nerve-fibres is the stratum of large ganglion cells, whose axons they 

 are; next to this the inner molecular layer, or inner synapse layer, 

 made up largely of the branching dendrires of these cells. The fifth 

 layer is the inner granular or nuclear layer, containing many fusiform 

 (bipolar) ' granule ' cells which send out axons into the fourth, and 

 dendrites into the sixth, or outer molecular layer, and are thus con- 

 nected with the ganglion cells of the third layer on the one hand, and 

 with the terminations of the rod and cone fibres of the seventh or outer 

 nuclear layer on the other. The arborizations of the axons of these 

 bipolar cells are situate at different 

 levels in the internal molecular 

 layer. The bipolar cells connected 

 with the rod fibres send their axons 

 right through the internal, mole- 

 cular layer to arborize around the 

 bodies of the ganglion cells, whereas 

 the axons of the bipolar cells con- 

 nected with the cone fibres ramify 

 about the middle of the layer 

 (Fig. 417). The seventh stratum 

 receives its name from the large 

 number of nuclei which it contains. 

 These belong to structures con- 

 tinuous with the rods and cones 

 of the ninth layer, which is divided 

 from the seventh by the external 

 limiting membrane. Each rod is 

 prolonged into the external nuclear 

 layer as a fine fibre, which has on 

 its course a swelling containing a 

 nucleus, and terminates (in mam- 

 mals) in a fine knob in the external 

 molecular layer among the den- 

 drites of the bipolar cells. Each 

 cone of the rod and cone layer is 



directly prolonged into a nucleated enlargement in the external nuclear 

 layer. From this enlargement a fibre (cone fibre) , of considerably greater 

 calibre (hi mammals) than the rod fibre, passes into the external mole- 

 cular layer, where it forms an arborization, which comes into relation 

 with the arborization of the dendrites of a bipolar cells. At the fovea 

 centralis the rods are entirely absent, and the other layers of the retina 

 greatly thinned; over the optic disc neither rods nor cones are present. 

 The disc is pierced by the retinal bloodvessels (Fig. 418'). 



External to the rods and cones is a sheet of pigmented epithelial cells 

 of hexagonal shape, belonging to the choroid, but remaining attached 

 to the retina when the latter is separated, and therefore often reckoned 

 as its most external layer. 



A little behind the cornea and anterior to the retina is the lens, en- 

 closed in a capsule, and attached to the choroid by the suspensory 

 ligament, or zonule of Zinn. The iris hangs down in front of the lens 

 like a diaphragm, with a central hole, the pupil. Incorporated in the 



Fig. 418. Retinal Bloodvessels (Henle). 

 The arteria centralis is seen issuing 

 from the optic dies and branching over 

 the retina. The shaded area in the 

 middle of the figure represents the 

 yellow spot with the fovea centralis in 

 its centre. 



