42 THE EYE. 



6. MULLER'S FIBERS OF THE RETINA. 



Genetically, the sustentacular fibers, or fibers of Miiller, in the 

 retina are, like the whole retina, of ectodermic origin, and repre- 

 sent a highly developed form of neurogliar tissue. They penetrate 

 the retina from within and extend as far as the inner segments of 

 the rods and cones. Each fiber represents a long, greatly modified 

 epithelial cell, terminating in one or more broad basal plates, which 

 come in contact with those of adjacent fibers, thus forming a sort 

 of membrane the internal limiting membrane. Owing to its 

 marked plasticity, each fiber presents certain peculiarities within 

 the various layers of the retina through which it penetrates. 

 Thus, within the molecular layers the fiber is provided with trans- 

 versely directed processes and platelets. Within the nuclear layers, 

 on the other hand, are numerous lateral indentations, which corre- 

 spond to the impressions produced by the cells of these layers. At 

 the inner surface of the cones and rods the fibers terminate in end- 

 plates, which represent cuticular formations, and, blending with one 

 another, form a single membrane the external limiting membrane. 

 This membrane is perforated by the rod-fibers and cone-fibers. The 

 end-plates of the fibers give off externally short, inflexible fibrils, 

 which form the fiber-baskets containing the basilar portions of the 

 inner segments of the rods and cones. (Vid. Fig. 361.) Miiller's 

 fibers do not appear as fibers in chrome-silver preparations, but as 

 complicated cellular structures, as above depicted. In preparations 

 of the retina, stained in a differential neuroglia stain (Benda's 

 method), clearly defined fibers, stained after the manner of neuroglia 

 fibers, may be differentiated. These fibers are in contact with or 

 are imbedded in the protoplasm of the Miiller's fibers. 



7. THE RELATIONS OF THE ELEMENTS OF THE RETINA TO 



ONE ANOTHER. 



We shall now take up the relationships existing between the 

 various elements of the retinal strata, giving the theories now 

 generally accepted and based on observations made with the Golgi 

 and methylene-blue methods, and more particularly on the investi- 

 gations of Ramon y Cajal (see diagram, Fig. 361) : 



1. The inner processes of the rod-visual cells end, as a rule, in 

 small expansions within the outer molecular layer, in which also the 

 processes of the cone-visual cells terminate in broader branched 

 pedicles. In this layer also are situated the terminal arborizations 

 of the dendrites and neuraxes of certain cells belonging to the inner 

 nuclear layer. 



2. The inner nuclear layer consists, as we have seen, (a) of bipolar 

 cells, which constitute the principal portion of this layer, () of hori- 

 zontally placed cells lying immediately beneath the outer molecular 

 layer, and (V) of the layer of spongioblasts situated at the junction 



