The distribution of rods and cones in the retina of the cat (Felis domesticus).

Abstract

AbstractThe distribution of rods and cones in the cat retina was studied by light microscopy. The rods and cones were counted from the area centralis to the temporal periphery in photomicrographs of transverse sections through the inner segments. A 16% correction for the effect of tissue shrinkage was applied to the densities obtained in fixed‐dehydrated tissue, by comparing these counts with those obtained from similar retinal areas in fresh tissue.The cone distribution was characterized by a steep increase in cone density centrally, which was elongated in the nasotemporal axis, and coincided approximately with the central increase in ganglion cell density (Stone, '65). Cone density in the area centralis peaked at 26,000–27,000/mm2, and fell to a plateau of 4000/mm2 in the periphery and to less than 3000/mm2 near the ora serrata. The density of rods was greater than the density of cones in all regions. The rods reached a maximum density of 460,000/mm2 at an eccentricity of 10–15°, in a region which completely surrounded the central cone elevation. Centrally, the rod distribution was characterized by a sharp fall in density, reaching a low of 275,000/mm2 at the point of peak cone density. Peripherally, there was a plateau of high rod density at above 400,000/mm2, out to about 30° temporal eccentricity. Beyond this plateau, rod density steadily fell, and reached a low of 250,000/mm2 near the ora serrata. The rod/cone ratio reached a low value of 10.5–11.0 in the central region of maximum cone density. It then rose steeply to reach a plateau of about 65 in the periphery, while near the ora serrata there were 100 rods for each cone.The rods were arranged in well defined rows, and each rod was surrounded by six other rods. The cones were interspersed throughout the rod mosaic and were less regularly arranged than the rods. In fresh tissue, rod outer segment diameters ranged from 1.0 μ to 1.6 μ. rod diameters were smallest in the region of maximum rod density and increased, along with the decrease in rod density, both in the periphery and in the area centralis. Two thirds of the central decrease in rod density from the pericentral peak could not be accounted for by the increase in the percentage of space occupied by cones, and was consistent with a central increase in rod diameter.Receptor‐ganglion cell convergence was estimated by comparing receptor densities with the ganglion cell densities of Stone ('65). Cone‐ganglion cell convergence reached a minimum in the area centralis at the cone density peak, probably indicating the retinal region in which photopic acuity is at an optimum. Minimum rod‐ganglion cell convergence, as estimated from the rod/large ganglion cell ratio, reached a minimum at about 7.5° central to the peak of rod density, and at about the same eccentricity as the human scotopic acuity optimum.