THE EYEBALL. 



743 



FIG. 164. 



cells fibres are continued to the fifth or internal granule layer, which consists 

 of granular cells with nuclei. Between the third and fifth layers is a layer 

 of vesicular matter containing nerve-fibrils of extreme minuteness. This 

 layer is the fourth or internal granulated or molecular layer. The sixth or 

 external granulated or molecular layer consists of parallel interlaced fibres, 

 containing nuclei and smooth cells. The seventh or external granule layer is 

 very similar to the fifth. The eighth layer consists of a delicate membrane 

 of connective tissue, called the membrana limitans externa. The ninth or 

 bacillary layer, or layer of rods and cones, or Jacob's membrane, is composed 

 of two elements, the rods and cones. These layers are supported by con- 

 nective tissue and a peculiar neuroglial structure known as the radial fibres 

 of Muller. (Fig. 163.) The rods are cylindrical bodies, each ending exter- 

 nally in a truncated, flattened extremity, and internally as an attenuated 

 fibre, which probably communicates with the deeper layer of ganglion cells. 

 The cones, as their name indicates, are conical-shaped bodies. Each consists 

 of two portions, a conical body having projecting from its apex a rod-like 

 segment, which appears in all respects like the rods. This segment is called 

 the cone rod. The terminal extremities of the cone rods do not extend as 

 far externally as the extremities of the rods. The rods and cones have 

 been demonstrated to consist of two segments or limbs, which are com- 

 posed of filaments, granular matter, and nuclei. The outer limb of the 

 rods contains a pinkish pigment known as " visual purple/' which is 

 extremely sensitive to light. 



The optic nerve, where it pierces the coats of the eye, projects somewhat 

 beyond the surface of the retina, as a papilla; here the essential nerve ele- 

 ments of the retina are absent, and luminous 

 rays are unperceived ; hence, it is called the 

 blind spot. About 2.6 mm. external to the 

 point of entrance of the optic nerve, and in 

 the exact centre of the retinal surface corre- 

 sponding to the antero-posterior axis of the eye, 

 is the " yellow spot of Sommerring," or macula 

 lutea. (Fig. 164.) It is an elliptical-shaped spot, 

 having its long diameter transverse. In the 

 centre of the macula lutea is a depression called 

 the fovea centralis. At this point the nervous 

 layer of the retina is very much modified in 

 the composition of the different layers. The 

 nervous layer is much thicker than at any other 

 part of the membrane. The ganglion (third) 

 and external granulated (sixth) layers are even 

 more thickened. The ganglion layer consists of 

 six or eight laminae of cells. The rods of the 

 ninth layer are absent, and are replaced by 

 cones. In the fovea centralis the internal gran- 

 ulated (fourth), the internal granule (fifth), and the optic nerve-fibre 

 (second) layers are wanting. The ganglion cell (third), the external gran- 

 ulated (sixth), and the external granule (seventh) layers are increased in 

 thickness. The ganglion layer of cells in the fovea consists of three 

 laminae. In all portions of the nervous layer the rods greatly predomi- 

 nate in number over the cones, excepting in the macula lutea, where they 

 are entirely absent. The retina is much thicker posteriorly, becoming thin- 

 ner as it extends forward, the nervous layer gradually disappearing in the 

 anterior portion of the membrane. 



The interior of the eyeball is divided into two portions by the crystalline 



Objects on the Inner Surface 

 of the Retina. In the centre of 

 the ball is the yellow limbus 

 luteus, here represented by shad- 

 ing, and in its middle the dark 

 spot. To the inner side is the 

 nerve, with its accompanying 

 artery. (After Sommerring.) 



