iv] Poxfn'itfnil or Senior;/ Area 103 



ground and offer some remarks on the varieties of common sensation believed to exist and to 

 contribute, either alone or in combination, to the registration of a conscious sensory perception. 

 I must also point out the parts of the cortex which are supposed to be concerned in the elaboration 

 and interpretation of these several kinds of sensations. 



The Tactile Sense 1 . 



It would appear at first sight that, since impairment of the primary component in the 

 mechanism of the tactile sense, the power of recognising cutaneous pressure, is such a 

 simple clinical phenomenon that the cortical area acting as a register for this particular sense 

 would be readily discovered. But how far this is from the truth is proved by the reality that 

 we are no nearer our goal to-day than we were twenty years ago, and that the question is 

 becoming more and more beset by bewildering theories. 



Avoiding as far as possible a discussion of different doctrines, we will enquire how far the 

 findings set forth in this research assist us in the localisation of this sense. Now, if the data 

 concerning the pathway for sensory impulses already detailed be correct, it necessarily follows 

 that the first part of the cerebral cortex to be influenced by a tactile impression must be 

 that of the postcentral gyrus, and particularly that on the Rolandic side, and in accordance 

 with our knowledge of other sensory areas we should expect that destruction of this part 

 would at least give rise to abolition of the power of recognising touch 2 . For proof on this 

 point, however, we are compelled to turn to the clinical observer; but, as I have already 

 stated, the evidence derived from observations on the human subject is, as it now stands, weak. 

 For, although clinicians have provided us with a lengthy array of cases in which unequivocal 

 sensory disorder of some kind has been found in association with lesions of the Rolandic or 

 "centro-parietal" (von Monakow) cortex, the information bearing on the sense which specially 

 forms the subject of the present discussion is admittedly insufficient. Looking around for an 

 explanation of this apparent breakdown in the clinical evidence, several suggestive circum r 

 stances come to mind : above all things we have to reflect on the r6le, in regard to registration, 

 played by the subordinate centres which the sensory impulse has to traverse in its upward 

 course, and in so doing take cognisance of the explanatory consideration exploited by Hitzig 

 that the tactile impression may be registered in subcortical centres before being passed on to 

 the brain surface for complete interpretation : or, modifying Hitzig's view, we have to consider 

 whether these intermediate stations do not exert some influence in dulling the intensity of the 

 stimulus, and so increasing the difficulty of the clinician in providing a positive demonstration 

 of tactile sensory disability from cortical lesion. Concerning this difficulty of recognition, 

 clinical observers seem unanimous in stating that extreme care must be exercised in the 

 examination of individuals for disorder of the tactile sense, and they also tell us that on 

 account of the fugacity or variant nature of the disability these examinations must be 

 frequently and patiently repeated. 



1 The tactile sense is made up of three components, the recognition of cutaneous pressure, the orientation of 

 the same, and the power of discriminating points in contact. Of these the first is undoubtedly the simplest, and 

 the second, because it involves a more complex psychic process, is higher than the third. 



2 After removal of the sensory centre of a dumb animal it is difficult to tell whether it feels or not and 

 impossible to say in what manner it feels, because our chief criterion of its power of tactile recognition is the 

 responsive muscle movement, and as Munk long ago indicated there is no knowing whether this is not of reflex 

 nature. Experiment, therefore, cannot assist us much in the elucidation of this problem. 



