. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 396 Comparative Animal Physiology fibers of the retina pass across the inner surface of the retina and enter the optic nerve. The region of exit of the nerve contains no rods and cones, is thereby insensitive to hght, and is known as the "bhnd ; In one region close to the optical axis of the eye the rods are fewer in number and the cones are much more numerous. This area is called the area centralis. In birds, in a few mammals, and in reptiles and fishes the area centralis is thinner than the surr
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 396 Comparative Animal Physiology fibers of the retina pass across the inner surface of the retina and enter the optic nerve. The region of exit of the nerve contains no rods and cones, is thereby insensitive to hght, and is known as the "bhnd ; In one region close to the optical axis of the eye the rods are fewer in number and the cones are much more numerous. This area is called the area centralis. In birds, in a few mammals, and in reptiles and fishes the area centralis is thinner than the surrounding retina, because of a decreased thick- ness of the layer of secondary and tertiary neurones, and is referred to as the fovea centralis. In the fovea of certain birds (, hawks, swallows, terns) these neuronal layers are not merely thinner, but their thickness va- «(»M.'' i lamina vitrea ^pigmeni epithelium receptor layer '^' (<"j<^~^ â 'â rr^- . %*.^'v,"'{' outer nuclear outer plexi- form layer % inner nuclear layer inner plexi- form layer. ganglion layer^ nerve fibers Fig. 110. Histology of the human retina. Left, vertical section through the retina in the nasal fundus; right, neuronal connections in the retina as revealed by silver impreg- nation methods (after Polyak). a, Amacrine cell; h, bipolar cells; c, cones; ch, centrifugal bipolar, believed to conduct outward through the retina; dh, diffuse bipolar; g, ganglion cells; h, horizontal cells; m, Miiller fibers, forming limiting membranes; pg, parasol gan- glion cell; r, rods. From Walls.^"' ries in such a way as to spread the image over a larger surface, thereby per- mitting a higher degree of visual acuity. This spreading lens effect is caused by the fact that the refractive index of the retinal tissue is greater than that of the vitreous body. The fovea of man is a broad area with very little if any spreading power. The two types of sense cells, rods and cones, are
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