A text-book of physiology for medical students and physicians . it would appear that the higher the animal isin the scale of development the larger is the number of fibers thatdo not cross in the chiasma. At least it is true that a larger num-ber remain uncrossed in man than in any of the mammalia, and it is 208 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. also possible or probable that the extent of decussation in manshows individual differences. There seems to be no acceptablesuggestion regarding the physiological value of this partial decus-sation other than that of a probable relation to binocula
A text-book of physiology for medical students and physicians . it would appear that the higher the animal isin the scale of development the larger is the number of fibers thatdo not cross in the chiasma. At least it is true that a larger num-ber remain uncrossed in man than in any of the mammalia, and it is 208 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. also possible or probable that the extent of decussation in manshows individual differences. There seems to be no acceptablesuggestion regarding the physiological value of this partial decus-sation other than that of a probable relation to binocular vision. Ithas been used to explain the physiological fact that simultaneousstimulation of symmetrical points in the two retinas gives us asingle visual sensation. The Projection or Localization of the Retina on theOccipital Cortex.—It would seem most probable that the pathsfrom each spot in the retina terminate in a definite region ofthe occipital cortex, and attempts have been made by variousmethods to determine this relation. According to Henschen.* the. Fig. 93.—Perimeter fields in quadrant hemianopia. The outline of the visual fieldsis given by the dotted lines. Blindness in the left upper quadrants; cortical lesion in andbelow the calcarine fissure (taken from Beevor and Collier;. visual paths in man end around the calcarine fissure on the mesial surface of the brain, and this portion of the occipital lobe should be regarded as the true cortical center for vision, the remainder of the occipital cortex being perhaps the seat of visual memories or associations. There seems to be much evidence, indeed, that the immediate ending of the optic paths lies in this region. Thus, Donaldsonf found, upon examination of the brain of Laura Bridgman, the blind deaf-mute, that the cuneus especially showed marked atrophy, and Flechsig,t by means of the myeliniza- tion method, arrived at the conclusion that the optic fibers end chiefly along the margin of the calcarine fissure. Clinic
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