The Outer limiting layer (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 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.