132 Visuo-Sensory and Visuo-P&ychic Areas [CHAP. 



clinical and pathological, and (3) the embryological and anatomical, and here let me say 

 that there is such a wealth of literature on this subject that a complete retrospect is 

 out of the question. Of many articles I have been able to procure only an epitomised 

 account, and I fear that some which may have appeared in the lesser known journals have 

 escaped notice altogether. 



A. 77(e Experimental Evidence. 



Much as we are indebted to experimental methods in helping us to advance 

 our knowledge of the topographical distribution of the cortical visual area, it is fairly 

 obvious that even the most ardent experimenter will refuse to maintain that the means 

 which have been employed hitherto, namely, ablation or destruction of given portions of the 

 brain, can possibly lead to a precise and exact definition of the area. The discrepancies, 

 contradictions, and want of accordance concerning the limits of the area, which appear in 

 the accounts presented by different experimenters, are sufficient proof of the dangerous 

 inaccuracy of ablation methods in determining the mere outside limits of the field; and if, 

 as we have reason for supposing is the case, the cortical visual area be divisible into districts, 

 each having a distinct function to perform, how much more hopeless it is for us to expect to 

 receive much assistance from experiment by ablation in effecting a territorial subdivision of 

 the area ! The obstacles which confront the experimenter are to a large extent anatomical 

 in kind ; if, for example, a removal of the field of cortex, bearing what I have called a 

 calcarine type of structure, were decided upon, and even supposing the operator to be 

 perfectly familiar with the anatomical distribution of that cortex, it would still be quite 

 impossible for him to ablate it without the contaminating influence of injury to extraneous 

 and contiguous structures. Thus, how is the cortex from the stem of the calcarine fissure 

 to be removed, first, without occlusion of the branch of the posterior cerebral artery 

 running therein and carrying the blood supply, not only to the convolutions forming the 

 calcarine walls, but also to those on both sides of the parieto-occipital fissure ? and secondly, 

 without opening into the posterior horn of the lateral ventricle ? And how, also, in the 

 destruction of any part of the visual cortex, is it possible to avoid injury to subjacent 

 strands of fibres, which may have an important physiological significance and yet not be 

 connected with the part which is being operated upon ? In addition to these difficulties, 

 serious enough in themselves, it is to be remembered that such experiments have to be 

 carried out on the lower animals, and that even in the most intelligent and suitable species 

 it is almost impossible to conduct the psychical analysis which is necessary before the 

 visual condition subsequent to operation can be interpreted, and a decision arrived at as 

 to whether the resulting disturbance is due to a primary blunting of the sensations or to 

 some secondary failure in their psychical appreciation. 



The foregoing considerations indicate what extreme caution must be exercised in drawing 

 conclusions from experiments alone, and unfortunately the same considerations apply to all 

 operations dealing with sensory areas. However, although our knowledge of cerebral localisa- 

 tion has advanced to such an extent that we are now able to pick holes in the observations 

 of earlier experimental workers, we never shall forget that the labours of these pioneers 

 in an extremely difficult branch of research have served as a guiding light, leading our 

 knowledge of cortical localisation to its present state. 



