. A manual of zoology. Zoology. GENERAL ORGANOLOGY 119 stages the visual cells are closely related to accumulations of pigment wliich occur either in or surrounding the cells. That this pigment is not absolutely essential for light perception is shown by the visual powers of albinos which are free from pigment, but it clearly must increase the sensitivity of the cells, for pigmentation is so common that the simplest eyes may be defined as sharply defined pigment spots, to which there is frequently added a lens to concentrate the light (fig. 84, III). Eyes.—From such beginnings, which are evide
. A manual of zoology. Zoology. GENERAL ORGANOLOGY 119 stages the visual cells are closely related to accumulations of pigment wliich occur either in or surrounding the cells. That this pigment is not absolutely essential for light perception is shown by the visual powers of albinos which are free from pigment, but it clearly must increase the sensitivity of the cells, for pigmentation is so common that the simplest eyes may be defined as sharply defined pigment spots, to which there is frequently added a lens to concentrate the light (fig. 84, III). Eyes.—From such beginnings, which are evidently only intended to recognize light and darkness, there are all transitions to the image-forming eyes of the vertebrates and apparently the cephalopods. The retina is rendered more efficient by the development of rhabdoms on the peripheral ends of the visual cells, rod-like processes which aid in hght perception, and in the vertebrates usually divided into rods and cones (fig. 85, 9).. P Fig. 85.—Human retina (after Gegenbaur). P, pigment-Ia3-er; E, layer of sensory cells; G, optic ganglion; i, limitans interna; 2, nerve-fibre layers; 3, ganglion cells; 4, inner reticular layer; 5, inner granular layer; 6, outer reticular layer; 7, outer granular layer; 8, limitans externa; 9, rods and cones; 10, tapetum nigrum; M, MuUer's fibres. In the vertebrates and many invertebrates the retina contains a reddish pigment, the 'visual purple,' which is quickly bleached in the light and as quickly regenerated in darkness, and which apparently pla3-s an important part in vision. In the course of the optic nerve there are numerous ganglion cells which form an optic ganglion (figs. 85 and 356), lying outside the eye in the invertebrates, in the vertebrates forming a number of layers (G, fig. 85), inside the retina proper (E), which is formed of the visual cells (outer granular layer) with the fibres of the rods and cones and the rhabdoms themselves. Accessory Structures.—If a sharp imag
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