608 PHYSIOLOGY CHAP. 



one occipital lobe produce bilateral visual disturbances which are 

 hemiopic in character ; the halves of the two retinae corresponding 

 to the side of the injured occipital lobe are blind (homonymous 

 bilateral hemianopsia). The perimetric observations made in 

 some of these cases show that the line of demarcation between the 

 blind and the seeing parts of the retina does not, as a rule, pass 

 through the fixation point, but to its blind side, i.e. the fovea is 

 not comprised in the hemiopic lesion. 



Clinical evidence in many cases seems to show that the visual 

 area of man is better defined and less diffuse than in monkeys, as 

 in the latter it is more restricted than in dogs. Not a few clinical 

 cases, moreover, indicate that it is lesions of the inner or mesial 

 surface of the occipital lobe which cause the most serious dis- 

 turbances of vision. 



Henschen (1892) maintained on the basis of his clinical and 

 anatomo-pathological researches that the visual area of the human 

 brain is confined to the cortex of the calcarine fissure, but critical 

 examination of the arguments on which he based this theory 

 shows that the visual centre cannot be contained within such 

 narrow limits. He has not cited a single case that is anatomically 

 sound, in which a lesion sharply limited to the calcarine area 

 produced total and permanent hemianopsia. In all cases so far 

 published of cortical hemianopsia there were more extensive 

 lesions, both of the mesial surface and of the convex surface of 

 the occipital lobe. 



According to Dejerine and Vialet (1893) the cortical visual 

 centre of man occupies the whole extent of the mesial face of the 

 occipital lobe, limited in front by the parieto-occipital fissure, 

 above by the upper border of the hemisphere, below by the lower 

 border of the third occipital gyrus, behind by the occipital pole. 

 But lesions of the cortex of the three external occipital convolutions 

 can also produce hemianopsia, as proved by Turner (1895) and 

 Pick (1896). Crispolti (1902) concluded from a critical survey of 

 155 clinical cases that the cuneus is of chief importance for vision, 

 the lingual and fusiform gyri of less, but that the cortex of the 

 outer surface of the occipital lobe, i.e. of the three occipital con- 

 volutions, is also part of the visual centre. 



Monakow came to the same conclusions (1897-1902) when 

 he referred the visual sphere of man to the three occipital con- 

 volutions, the entire cuneus, the lingual lobule, and the descending 

 gyrus, in addition to the calcarine region which forms its most 

 important part. Bernheimer (1900), with the myelination 

 method, arrived at the same conclusions. Flechsig (1901), by the 

 same method, located the central focus of vision in the calcarine 

 fissure, and the margins of the cuneus, the lingual lobule, and the 

 cortex of the external occipital pole ; but he admitted that it also 

 extended beyond these limits (V, Figs. 299, 300). 



