A text-book of physiology for medical students and physicians . the cones. Lastly, in the eye ofthe totally color blind, in the dark-adapted eye in dim lights, in thecolor-blind peripheral area of the normal eye, and in the eyes ofmost distinctly night-seeing animals, such as the mole and the owl,vision seems to be effected solely by the rods. These facts findtheir simplest explanation perhaps in the view advocated by Pari-naud, Franklin, von Kries,* and others, according to which theperception of color is a function of the cones alone, while the rodsare sensitive only to light and darkness, a
A text-book of physiology for medical students and physicians . the cones. Lastly, in the eye ofthe totally color blind, in the dark-adapted eye in dim lights, in thecolor-blind peripheral area of the normal eye, and in the eyes ofmost distinctly night-seeing animals, such as the mole and the owl,vision seems to be effected solely by the rods. These facts findtheir simplest explanation perhaps in the view advocated by Pari-naud, Franklin, von Kries,* and others, according to which theperception of color is a function of the cones alone, while the rodsare sensitive only to light and darkness, and by virtue of their powerof adaptation in the dark through the regeneration of their visualpurple, they form also the special apparatus for vision in dim * Von Krie«, Zeitschrift f. Psyehologie u. Physiol, d. Sinnesorgane, 9,81, 1895. 23 354 THE SPECIAL SENSES. lights (night vision). Color blindness, therefore, whether total orpartial, may be regarded as an affection or lack of normal develop-ment of the cones. On the other hand, those interesting cases in. Fig. 152.—Schema of the structure of the human retina (Greeff): I, Pigment layer;//, rod and cone layer; ///, outer nuclear layer; IV, external plexiform layer; V, layerof horizontal cells; VI, layer of bipolar cells (inner nuclear); VII, layer of amacrinal cells(without axons); VIII, inner plexiform layer; IX, ganglion cell layer; X, nerve fiberlayer; 6, fiber of Muller. which the vision, while good in daylight, is faulty or lacking in dimlights (night blindness, hemeralopia) may be referred to a defectivefunctional activity of the rods, probably from lack of formation ofvisual purple. Theories of Color Vision.—A number of theories have beenproposed to explain the facts of color vision. None of them hasbeen entirely successful in the sense that the explanations it affords PROPERTIES OF THE RETINA. 355 have been submitted to satisfactory experimental verification. Theimmediate stimuli that give rise to the visual impu
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