. Elements of human physiology. Physiology. SPECIAL SENSES 565 been kept in the dark, on the other hand, the cells of the pig- ment-layer are quite flat, so that the front part of the retina, including the rods and cones, can be removed without any difficulty (Fig. 256). Thus tiie function of the pigmented epithelium is to supply visual purple to the outer limbs of the rods as fast as the pigment already there is bleached by light. It might be thought that this chemical change was the active agent in producing excitation of the optic nerve- fibres ; but the facts that in the fovea centralis, t
. Elements of human physiology. Physiology. SPECIAL SENSES 565 been kept in the dark, on the other hand, the cells of the pig- ment-layer are quite flat, so that the front part of the retina, including the rods and cones, can be removed without any difficulty (Fig. 256). Thus tiie function of the pigmented epithelium is to supply visual purple to the outer limbs of the rods as fast as the pigment already there is bleached by light. It might be thought that this chemical change was the active agent in producing excitation of the optic nerve- fibres ; but the facts that in the fovea centralis, the region of Fig. Three types of retinal variiition obtained on exposure to light (Waller). I, fresh retina. 11, same retina after two hours. Ill, same retina after twenty-four hours. The black line at the lower border of each record marks the period of exposure to lioht. most distinct vision, we iind only cones which contain no visual purple, and that in certain birds there are no rods and no visual purple in the whole retina, show that this chemical process, interesting though it may be, is not essential for the conversion of light-waves into a nervous impulse. When light falls upon the retina the cones are reti-acted, and lie close upon the external limiting membrane ; whereas in an eye that has been kept in the dark they extend down between the rods as far as the pigmented Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Starling, Ernest Henry, 1866-1927. Chicago : W. T. Keener
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