210 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



ber remain uncrossed in man than in any of the mammalia, and it is 

 also possible or probable that the extent of decussation in man 

 shows individual differences. There seems to be no acceptable 

 suggestion regarding the physiological value of this partial decus- 

 sation other than that of a probable relation to binocular vision. It 

 has been used to explain the physiological fact that simultaneous 

 stimulation of symmetrical points in the two retinas gives us a 

 single visual sensation. 



The Projection or Localization of the Retina on the 

 Occipital Cortex. It would seem most probable that the paths 

 from each spot in the retina terminate in a definite region of 

 the occipital cortex, and attempts have been made by various 

 methods to determine this relation. According to Henschen,* the 



Fig. 93. Perimeter fields in quadrant hemianopia. The outline of the visual fields 

 is given by the dotted lines. Blindness in the left upper quadrants; cortical lesion in and 

 below 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 terminus of the optic fibers. 

 There seems to be much evidence, indeed, that the immedi- 

 ate ending of the optic paths lies in this region. Thus, 

 Donaldson f found, upon examination of the brain of Laura 

 Bridgman, the blind deaf-mute, that the cuneus especially 

 showed marked atrophy, and Flechsig, J by means of the myeliniza- 

 tion method, arrived at the conclusion that the optic fibers end 

 chiefly along the margin of the calcarine fissure. Clinical cases 

 are frequently quoted in which lesions of the region of the calcarine 

 fissure were followed by a more or less complete hemiopia. When, 

 as seems to be the most common occurrence, such lesions occur 

 above the fissure, in the cuneus, or below the fissure, in the gyrus 

 lingualis, the resulting hemiopia is confined to corresponding 



* Henschen, "Brain," 1893, 170. 



t Donaldson, "American Journal of Physiology," 1892, 4. 



I Flechsig, "Localisation der geistigen Vorgange," "Leipzig, 1896. 



