v] ('finical <ntd P(itli<>I(i(/ic<d Evidence regarding Function 135 



which remains after destruction of the occipital lobes is attributable to the activity of 

 these subcortical centres. But be it noted, in man and in the higher animals, the pulvi- 

 uaria and external geniculate bodies arc much more important than the anterior corpora 

 quadrigemina, for while injury to the former produces marked effects, the latter bodies have 

 been destroyed without any resulting impairment of light or colour vision. 



The question of the restitution of vision after removal of the occipital cortex is one 

 to which the older experimenters gave much attention, and recently the point has been 

 revived by Vitzou, who reports an extraordinary case in a young ape, in which a complete (?) 

 removal of the occipital lobes resulted in total blindness. This blindness, however, began 

 to pass off at the end of a few months, and at the end of two years and two months 

 vision was apparently restored. Operating again he found the gap filled up with what 

 he took to be nervous tissue, and on removing this he finally rendered the animal 

 permanently and totally blind. To explain this remarkable case Vitzou resorts to the 

 regeneration or rather the neoformatiou hypothesis, but it seems more probable, as Bolton 

 suggests, that at the primary operation the anterior parts of the visual area, which extend 

 forward in advance of the splenium of the corpus callosum, were left intact and afforded 

 the animal a certain amount of visual power. 



Having alluded briefly to these side-issues in the cortical representation of vision, we 

 may sum up the situation by saying, that experimentation on the ape, dog, and other animals 

 proves that there exists in the occipital lobe a field of cortex removal of which is followed 

 by blindness, but that a definition of the exact limits of this area is beyond the scope of 

 the experimenter. 



B. Tlie Clinical and Pathological Evidence. 



Taking another view of the subject we have now to ascertain in what respects the 

 clinical can be brought into line with the experimental evidence, to what extent clinical 

 cases have assisted us to arrive at a precise orientation of the cortical visual area in the 

 human brain, and how far they warrant a subdivision of the area. 



Considering that well over 200 cases of cortical or subcortical hemianopsia, in which the 

 diagnosis has been verified by post-mortem examination, have been recorded in the literature 

 since Chaillou published his first case, in 1863, and that in spite of this accumulation of 

 knowledge, not only are many pathological disorders of vision far from being understood 

 clearly, but also the very confines of the area concerned with sight are not yet decided, we 

 may take it for granted, first, that the processes concerned with vision in the human 

 subject are of inconceivable complexity, and secondly, that natural lesions are no better 

 adapted than experimental for the exact determination of the limits of the area. 



The chief reason why a study of lesions in the human brain cannot be productive of 

 results which will lead to an accurate determination of topographic distribution again turns 

 on anatomy. Of course the cases studied in this connection have been cases of cerebral 

 softening, tumour, haemorrhage, or abscess, and of these, cases of cerebral softening have 

 constituted by far the majority. Now when we examine the blood supply of the convo- 

 lutions on the inner and posterior surfaces of the occipital lobe, we soon discover reasons 

 why cases of arterial thrombosis or embolism should be inadequate for purposes of exact 

 cortical localisation; we find that all these convolutions are supplied by branches derived 

 from one common trunk, the occipital artery of Duret, which in its turn springs from the 



