186 University of California PiiUications in A7iatomy l^'^^- 2 



regions of the hemisphere, e.g., of the frontal lobe, since that part of 

 the visual radiation will be pressed against or pulled onto the pos- 

 terior margin of the falx and to the tentorium.^ ^ (There is no need 

 therefore of supposing a "wide" representation of the visual function 

 in the hemisphere.) Later on, when the pressure or pathological 

 process extends into the more caudally situated portions of the striate 

 area and to the visual radiation beneath, incomplete or a complete 

 homonymous hemianopsia will develop ; 



(6) the peculiar arrangement of the upper and lower "peri- 

 pheral" segments of the visual radiation, appearing on cross sections 

 through the hemisphere as the upper and lower horizontal branches 

 of the external sagittal layer, with the macular perpendicular branch 

 between them, renders possible a fairly isolated injury of either of the 

 above mentioned branches, followed by loss of the particular extra- 

 macular or macular quadrants or of the entire homonymous halves of 

 the fields of vision. Yet the same position of the visual bundles will 

 not readily allow the interruption of both upper and lower horizontal 

 branches of the radiation by a single traumatic injury. For instance, 

 there could hardly occur gunshot injury damaging both of the hori- 

 zontal branches without some injury to the macular branch. The only 

 possible course of such an injury which would abolish both extra- 

 macular quadrants of the visual fields and yet spare macular vision 

 would be a gun shot channel perpendicular to the horizontal plane of 

 the hemisphere and a little away from the falx cerebri yet medially 

 from the plane of the perpendicular branch. (That this would hardly 

 be consistent with the preservation of life is not necessary to point 

 out.) The macular vision will be affected if a gunshot channel has 

 a fairly exact horizontal direction from outside inward, more or less 

 perpendicular to the long axis of the hemisphere, interrupting the 

 perpendicular branch. Since it would probably injure the fissura cal- 

 carina itself in its more oral portion there would at the same time be 

 an impairment of the extramaeular portions of the visual fields in the 

 form of "peripheral," perhaps multiple scotomata of various forms 

 with a portion of the extra-macular visual fields spared. Such a 

 complicated impairment of the visual fields proved indeed puzzling 



13 Such or a similar mechanism might account for the crescentie hemianopsia 

 in some eases reported by Allen (1930), where the seat of the tumor was on the 

 external face of the occipito-parietal lobes. Compression of both the upper and 

 lower horizontal branches of the visual radiation against the falx cerebri and 

 tentorium, and squeezing of both lips of the calcarine fissure (anterior portion), 

 while the perpendicular branch situated deep in the hemisphere escapes at first 

 from the effect of such a pressure, could cause such an hemianopsia. 



