r 



CEREBRAL LOCALIZATION 685 



others, have shown that in the monkeys the ablation of the occipital 

 lobes produces a permanent and total blindness. This result has 

 also been obtained by Panichi (1895), with this difference, however, 

 that the blindness can only be made permanent by extending the 

 ablation somewhat beyond the commonly accepted boundaries of the 

 occipital lobes. 



With the exception of certain minor details, the visual center may, 

 therefore, be said to be situated in the occipital realm of the cerebrum, 

 and this conclusion is well borne out by the defects following the 

 extirpation of only one of these lobes. Under this condition we obtain 

 a blindness which is confined to the corresponding halves of the retinae, 

 in other words, a bilateral hemianopia. The term of hemianopsia 

 may also be used to indicate this condition, because it refers to a loss 

 of vision in one-half of each visual field, while the former more directly 

 applies to a loss of function of one-half of each retina. 



The results of this operation, however, differ somewhat in different 

 animals, but this should not surprise us, because attention has already 

 been called to the fact that the fibers emerging through the optic 

 nerve, do not pursue a uniform course. We have seen that they 

 cross the mid-line completely in some animals and only partially in 

 others. In the first instance, the ablation of the occipital cortex of 

 one side must, of course, lead to a total blindness in the opposite eye. 

 It seems advisable, however, not to extend this discussion unduly, 

 but to confine ourselves to the conditions met with in man. We find 

 here that the destruction of one occipital lobe is followed by disturb- 

 ances in vision of hemiopic character, i.e., by a bilateral homonymous 

 hemianopsia. Thus, an injury to the left center produces a blindness 

 in the outer half of the left and the inner half of the right eye, and a 

 loss of vision in the opposite half of the visual field of each eye. 

 Quite similarly, a lesion affecting the right centef causes bhndness in 

 the two right halves of the retinae and left halves of the visual fields. 

 This implies that the crossing of the retinal fibers is about equal. It 

 is to be emphasized, however, that the foveae centrales are not involved, 

 and hence, the field of direct and most acute vision is always excepted 

 (Fig. 319). This peculiarity is explained by saying that the fovea cen- 

 trahs of each eye is connected with both centers, i.e., the foveae are 

 bilaterally represented.^ 



Very pecuhar types of blindness result if the lesion is situated in the 

 course of the fibers connecting the retinae with the cortical center for 

 vision. Thus, it must be evident that the destruction of one optic 

 nerve must lead to a total blindness in the corresponding eye, while a 

 lesion situated in the chiasma must produce bilateral defects in ac- 

 cordance with its location and extent. In a similar way, it may be 

 inferred that the destruction of the central optic tract posterior to 

 the chiasma must give rise to a hemianopia in the corresponding halves 



^ Sachs, Der Hinterhauptlappen, Leipzig, 1892; also: Laqueur and Schmidt, 

 Virchow's Archiv, clviii, 1900, 466. 



i^ (/' 



