

IE TUNICS OF THE EYE 1017 



inner processes of the rod bipolars run through the inner plexiform layer and arborize around 

 the bodies of the cells of the ganglionic layer; their outer processes end in the outer plexiform 

 layer in tufts of fibrils around the button-like ends of the inner processes of the rod granules. 

 The inner processes of the cone bipolars ramify in the inner plexiform layer in contact with the 

 dendrites of the ganglionic cells. 



The horizontal cells lie in the outer part of the inner nuclear layer and possess somewhat 

 flattened cell bodies. Their dendrites divide into numerous branches in the outer plexiform 

 layer, while their axons run horizontally for some distance and finally ramify in the same layer. 



The amacrine cells are placed in the inner part of the inner nuclear layer, and are so named 

 because they have not yet been shown to possess axis-cylinder processes. Their dendrites undergo 

 extensive ramification in the inner plexiform layer. 



5. The outer plexiform layer is much thinner than the inner; but, like it, consists of a dense 

 net-work of minute fibrils derived from the processes of the horizontal cells of the preceding layer, 

 and the outer processes of the rod and cone bipolar granules, which ramify in it, forming arboriza- 

 tions around the enlarged ends of the rod fibers and with the branched foot plates of the cone 

 fibers. 



6. The outer nuclear layer or layer of outer granules, like the inner nuclear layer, contains 

 several strata of oval nuclear bodies; they are of two kinds, viz.: rod and cone granules, so 

 named on account of their being respectively connected with the rods and cones of the next layer. 

 The rod granules are much the more numerous, and are placed at different levels throughout 

 the layer. Their nuclei present a peculiar cross-striped appearance, and prolonged from either 

 extremity of each cell is a fine process; the outer process is continuous with a single rod of the 

 layer of rods and cones; the inner ends in the outer plexiform layer in an enlarged extremity, and 

 is imbedded in the tuft into which the outer processes of the rod bipolar cells break up. In its 

 course it presents numerous varicosities. The cone granules, fewer in number than the rod 

 granules, are placed close to the membrana limitans extema, through which they are continuous 

 with the cones of the layer of rods and cones. They do not present any cross-striation, ,but con- 

 tain a pyriform nucleus, which almost completely fills the cell. Fr,om the inner extremity of the 

 granule a thick process passes into the outer plexiform layer, and there expands into a pyramidal 

 enlargement or foot plate, from which are given off numerous fine fibrils, that come in contact 

 with the outer processes of the cone bipolars. 



7. The Layer of Rods and Cones (Jacob's membrane). The elements composing this layer are 

 of two kinds, rods and cones, the former being much more numerous than the latter except in 

 the macula lutea. The rods are cylindrical, of nearly uniform thickness, and are arranged per- 

 pendicularly to the surface. Each rod consists of two segments, an outer and inner, of about 

 equal lengths. The segments differ from each other as regards refraction and in their behavior 

 toward coloring reagents; the inner segment is stained by carmine, iodine, etc.; the outer segment 

 is not stained by these reagents, but is colored yellowish brown by osmic acid. The outer segment 

 is marked by transverse stria;, and tends to break up into a number of thin disks superimposed 

 on one another; it also exhibits faint longitudinal markings. The deeper part of the inner seg- 

 ment is indistinctly granular; its more superficial part presents a longitudinal striation, being 

 composed of fine, bright, highly refracting fibrils. The visual purple or rhodopsin is found only 

 in the outer segments. 



The cones are conical or flask-shaped, their broad ends resting upon the membrana limitane 

 externa, the narrow-pointed extremity being turned to the choroid. Like the rods, each is made 

 up of two segments, outer and inner; the. outer segment is a short conical process, which, like 

 the outer segment of the rod, exhibits transverse strise. The inner segment resembles the inner 

 segment of the rods in structure, presenting a superficial striated and deep granular part, but 

 differs from it in size and shape, being bulged out laterally and flask-shaped. The chemical 

 and optical characters of the two portions are identical with those of the rods. 



Supporting Frame-work of the Retina. The nervous layers of the retina are connected together 

 by a supporting frame-work, formed by the sustentacular fibers of Muller ; these fibers pass 

 through all the nervous layers, except that of the rods and cones. Each begins on the inner surface 

 of the retina by an expanded, often forked base, which sometimes contains a spheroidal body 

 staining deeply with hematoxylin, the edges of the bases of adjoining fibers being united to form 

 the membrana limitans interna. As the fibers pass through the nerve fiber and ganglionic layers 

 they give off a few lateral branches; in the inner nuclear layer they give off numerous lateral 

 processes for the support of the bipolar cells, while in the outer nuclear layer they form a net- 

 work around the rod- and cone-fibrils, and unite to form the membrana limitans externa at the 

 bases of the rods and cones. At the level of the inner nuclear layer each sustentacular fiber 

 contains a clear oval nucleus. 



Macula Lutea and Fovea Centralis. In the macula lutea the nerve fibers are wanting as a 

 continuous layer, the ganglionic layer consists of several strata of cells, there are no rods, but 

 only cones, which are longer and narrower than in other parts, and in the outer nuclear layer 

 there are only cone-granules, the processes of which are very long and arranged in curved lines. 

 In the fovea centralis the only parts present are (1) the cones; (2) the outer nuclear layer, the 



