742 



SIGHT. 



tends from the entrance of the optic nerve to a point where the annular 

 fasciculi of the ciliary muscle are found ; at this point the nervous elements 

 cease to exist, and the layer has an irregular dentated margin called the 

 ora serrata. Beyond this the nervous layer is continued as a mere fibrous 

 extenuation. 



The optic nerve pierces the sclerotic and the choroid coats, and the pig- 

 mentary membrane of the retina, when it rapidly divides into vast numbers 

 of fibres, which consist alone of the axis-cylinders or their ultimate fibrillse. 



FIG. 161. 



FIG. 162. 



FIG. 163. 



FIG. 161. Diagrammatic Representation of the Connections of the Nerve-fibres in the Retina. 

 1, membrana limitans interna ; 2, optic nerve-fibre layer ; 3, layer of ganglion cells ; 4, internal 

 granulated or molecular layer ; 5, internal granule layer ; 6, external granulated or molecular 

 layer; 7, external granule layer; 8, membrana limitans exterior; 9, bacillary layer, or layer of 

 rods and cones. 



FIG. 162. Rod and Cone from the Retina of Man, preserved in a two per cent, solution of 

 perosmic acid to show the fine fibres of the surface, and the different lengths of the internal seg- 

 ment. The outer segment of the cone is broken up into disks, which, however, are still adherent 

 to one another ; at the base of the cone are seen a few fine hairs. ( + 1000 diameters.) 



FIG. 163. Diagrammatic Representation of the Connective Tissue and Radial Fibres of Miiller 

 of the Retina as seen near the Ora Serrata. The numbers correspond to those of the several 

 layers of the retina shown in Fig. 161. 



This layer of fibres is continuous over nearly the whole of the internal sur- 

 face, and is called the second or optic nerve-fibre layer. On its internal sur- 

 face, between it and the hyaloid membrane, is a delicate structure called the 

 first layer, or membrana limitans interna. The third or ganglion layer is com- 

 posed of multipolar ganglion cells, similar to those found in the cerebral 

 substance. In the posterior portion of the retina these ganglion cells are in 

 several layers ; at the macula lutea there are as many as eight, and at the 

 anterior portion of the retina there is but a single layer. From each of these 



