v] Anatomical Er'nlcncv reyanliny Function 139 



Alexia. 



The next variety <>t' psychical visual detect to which I shall refer has been described 

 by Kussniaul as " Wortblindheit," by Dejerine as ' Cecite verbale pure," by Wernicke as 

 " subcortical alexia." and by others simply as "alexia"; the condition is one which has 

 been widely recognised, and. briefly defined, it consists of an inability to comprehend written 

 or printed language, although the letters may be clearly seen and vision generally may be 

 unimpaired. 



Clinically, many degrees and varieties of this affection may appear, but fortunately there 

 is almost unanimous agreement concerning its pathological anatomy. In ten or more cases 

 which have been carefully examined (those of Monakow, Redlich, Verrey, etc.) the surface lesion 

 has been confined to the region of the left angular gyrus and the second occipital convolu- 

 tion, and usually has spread sufficiently deeply into the underlying white substance to 

 include the fasciculus longitudinalis inferior, an important band of fibres the connections of 

 which we shall have to mention presently. And although, in some instances, other bands, viz., 

 the fasciculus longitudinalis superior, the occipito-thalamic radiations of Gratiolet, the forceps 

 major, and in a few cases fibres pertaining to the splenium of the corpus callosum have 

 been involved, the stress of the injury seems always to have fallen on the band first alluded 

 to. Also it is becoming an established doctrine that destruction of the cortex in the region 

 of the angular gyrus by itself, or of the fasciculus longitudinalis inferior by itself, is insufficient 

 to produce alexia ; the two must go together, and for the production of the clinical manifesta- 

 tion long and short systems of association fibres necessarily must be destroyed. 



Colour Blindness. 



A few cases have been recorded in which colour blindness, or more correctly speaking 



hemi-achromatopsia, has followed a circumscribed lesion in the occipital lobe, and in three 



of these, published by Henschen, Yerrey, and Dejerine, the destruction has been confined, 



more or less, to the hinder end of the lingual lobule : hence it seems likely that the cortex in 

 this situation is associated with colour perception. At the same time we must be cautious 



in arriving at any definite conclusion on this point, because in all these cases the inability 

 to recognise colours has been complicated by hemiopia, or other visual defect. 



Optic aphasia is another variety of mind blindness and was described first by Freund. 

 An individual so affected, in spite of unimpaired vision and intellectual faculty, and notwith- 

 standing that he may recognise objects presented to his vision, is yet unable to give them 

 their correct name without calling into play other senses, such as that of touch. 



From the few cases that have been published we gather that the lesion underlying this 

 condition is to be sought in the white substance of the left occipital lobe, but pure cases, 

 that is, cases uncomplicated by other visual defect, are evidently uncommon. 



C. The Anatomical and Embryoloyical Evidence. 



When the anatomical evidence which we have now collected is given full consideration, 

 I think it will be granted that this is the only path by which we may hope finally to 



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