1466 



HUMAN ANATOMY. 



FIG. 1222. 



cells are separated by considerable intervals. Their axones, or axis-cylinder pro- 

 cesses, pass inward and become the nerve-fibres of the fibre layer. Converging toward 

 the optic entrance, they become consolidated into the optic nerve and pass to the 

 brain. The dendrites of the ganglion cells, one to three in number, run outward into 



the inner plexiform layer and end as richly branched 

 arborizations. These, like those of the amacrine cells, 

 terminate either diffusely, or in horizontal ramifica- 

 tions limited to definite strata, in connection with the 

 centrally directed processes from the bipolar cells. 



The nerve-fibre layer is composed almost 

 entirely, but not exclusively, of the axones of the 

 ganglion cells of the preceding layer. The individual 

 fibres, from .005-. 05 mm. in diameter, are collected 

 into bundles of varying size, which take a horizontal 

 course and converge toward the optic disc. They are 

 normally devoid of medullary sheaths, but acquire 

 them after passing through the lamina cribrosa of the 

 sclera. A few of the fibres are centrifugal, arising 

 from ganglion cells within the brain, and terminate 

 apparently in connection with the association amacrines 

 of the inner nuclear layer. 



In the macular region, the nerve-fibres are prac- 

 tically absent, those from the retinal area lying directly 

 to the temporal side of the macula arching above 

 and below the yellow spot. From the macula itself, 

 a special strand, known as the maculo -papillary bundle 

 and composed of about twenty-five fasciculi, passes 

 directly to the nerve-disc. 



The sustentacular tissue, the neuroglia of the 

 retina, exists in two forms as \hefibres of M'uller and 

 as the spider cells. 



The fibres of Miiller are modified neuroglia 

 fibres which pass vertically from the inner surface of 

 the retina through the succeeding layers as far as the 

 bases of the rods and cones (Fig. 1222). The inner 

 extremities of the fibres possess conical expansions, 

 which are in apposition and form an incomplete sheet, 

 known as the membrana limitans internet . As the fibres traverse the retinal layers, 

 they give off delicate lateral offshoots, which break up into a fine supporting reticu- 

 lum. Within the inner nuclear layer each fibre presents a broad expansion, in which 

 is situated the oval nucleus of the sustentacular cell, the fibre of Miiller. After 

 traversing the outer nuclear layer their broadened peripheral ends come into contact 

 and form a continuous sheet, the membrana limitans externa. From the latter deli- 

 cate offshoots continue outward and embrace the bases of the individual rods and 

 cones. In addition to the robust fibres of Miiller, neuroglia cells, in the form of 

 spider cells, are found in the nerve-fibre and ganglion cell layers. These cells send 

 out long delicate processes which extend between the processes and cells and thus 

 help to support them. 



The Macula Lutea. The structure of the retina undergoes important modifi- 

 cations in two areas, at the macula lutea and at the ora serrata. In the former the 

 n-an^-lion cells increase rapidly in number as the macula is reached, so that instead 

 of forming a single layer they are distributed in from eight to ten strata. The inner 

 nuclear layer is also increased in thickness. Within the fovea centralis, however. 

 in order to reduce to a minimum the layers traversed by the light-rays, the cerebral 

 layers are almost entirely displaced, only the absolutely essential retinal strata the 

 pigment cells and the visual cells with their necessary connections being retained 

 within the area of sharpest vision (Fig. 1223). On approaching the fovea, the 

 Uansjion cells rapidly decrease in number, until at the centre of the depression, they 

 Bi entirely absent and the nerve-fibre layer, therefore, disappears. The bipolar 



Supporting fibres of Miiller from 

 retina of ox; Golgi preparation. 

 (Cajal.) 



