v] Anatomical Evidence regarding Function 143 



Taken altogether there can be little doubt that these long tracts play an important 

 part as adjuvants in the elaboration and interpretation of visual impulses received by the 

 occipital lobe. 



We will pass on now to those observations which have aimed at the localisation of 

 the visual area in the human brain from the purely histological point of view, and under 

 this heading the research which first calls for notice is that published by Dr Bolton. 



Examining in serial sections the occipital lobes of the brains of five cases of old- 

 standing total blindness and one of anophthalmos, and using as a criterion the atrophy and 

 disappearance of certain nerve cell and fibre elements, Dr Bolton was able to map out with 

 the greatest distinctness an area which corresponds in all important respects with that 

 which clinico-pathological, developmental, and experimental researches have indicated as the 

 visuo-sensory area. 



Stated briefly, the histological alterations which served him as a restrictive guide, were 

 a diminution in thickness of the line of Gennari by almost 50 per cent, and of the outer 

 granule layer of nerve cells by almost 10 per cent. 1 , and I can personally testify to the 

 extreme thoroughness of his work, because his observations were begun in the laboratory over 

 which I have control, and as he progressed I had numerous opportunities of seeing his 

 preparations and of judging his methods of investigation. 



It will be unnecessary for me to describe the limits of the area defined by Dr Bolton, for, 

 on referring to his diagrams and descriptions, it may be found that if I did so, I should 

 merely reiterate what I have said already regarding the limits of my visuo-sensory area ; 

 but as we agree that the area over which the line of Gennari is distributed represents 

 the cortical visuo-sensory centre, it obviously follows that it is unnecessary to resort to patho- 

 logical material for a definition of the field, and as a matter of fact it can be determined in 

 a normal brain even without the aid of a microscope. 



It is to be regretted that Dr Bolton did not add to his research the examination of a 

 brain from a case in which the blindness had not been of such long-standing, for manifestly the 

 gross alterations which he has noted partake of the nature of secondary atrophy the form 

 of atrophy to which Gudden drew attention years ago and it would be interesting not only 

 to study the phases of atrophy exhibited by the affected cells, but also to acquire more 

 precise knowledge concerning the particular cells which are involved in the process, and 

 specially to learn whether we should be correct in assuming that the laminar thinning, which 

 Dr Bolton describes, is to a great extent ascribable to an atrophy of the large stellate cells 

 situated above and below the line of Gennari, along with their numerous long processes. 



Also it is disappointing to find that Dr Bolton says so little about the visuo-psychic area, 

 for, although he clearly recognises and indeed accurately figures the type of cell-lamination 

 in the parts bordering on the visuo-sensory area, he makes no attempt to define the limits of 

 the field bearing this type and dismisses the subject with the remark that " in the visuo- 

 psychic region surrounding the area of special lamination, old-standing optic atrophy causes 

 no modification of the lamination." But my histological observations compel me to doubt whether 

 this remark is wholly true, for I strongly suspect, as a result of the "partial" examination 



1 Boltou's outer granule layer of nerve cells corresponds to the lower part of S. Kamon y CajaFs layer 3. 



