Henry Gray (18251861). Anatomy of the Human Body. 1918.
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 arborizations 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 externa, through which they are continuous with the cones of the layer of rods and cones. They do not present any cross-striation, but contain a pyriform nucleus, which almost completely fills the cell. From 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 (Jacobs 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 perpendicularly 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 striæ, 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 segment 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 limitans 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 striæ. 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 Müller; 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 network 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