102 Remarks on Common Sensation [CHAP. 



sensibility if it involves the parietal side of that area, and for this reason it is permissible to 

 assume that the so-called " accidental " sensory phenomena have not been the product of 

 chance, but due to implication of the postcentral gyrus and its connected system of fibres 1 . 



Setting aside this question, it has been thoroughly proved, not only that the cortical localisa- 

 tion of sensation is attended with extreme difficulty, but that the detection of an impairment 

 of sensibility in consequence of a cortical lesion is no easy matter. To explain the relatively 

 slight sensory disorder in comparison with the severe motor paralysis which follows lesions of 

 the central gyri, the unlikely hypothesis has been set up that the sensory zone must be more 

 widely distributed than the motor, but to those who support this view the condition of affairs 

 attending spinal lesions might be pointed out. A minute focus of inflammation situated in the 

 anterior cornu of a portion of one spinal segment will suffice to produce an amount of muscular 

 atrophy which is unfortunately too plainly visible and permanent ; destroy, on the other hand, 

 one posterior root ganglion, and there will be no complete anaesthesia and only a temporary 

 and trifling diminution of sensation, so little that it will take all the skill of a practised ob- 

 server to detect it. In these two instances, instead of the cause and effect being unequal, it 

 is far likelier that the effect is more readily discerned in the one case than the other; so it 

 may be also in the case of the central gyri; a lesion sufficient in extent to produce paralysis 

 of a group of muscles may not be adequate to the production of a discernible equivalent 

 in the shape of sensory disorder. Apart from this, we have to take into consideration a 

 difference in the arrangement of components of the respective neuronic systems ; the motor 

 chain is composed of two links with but one interruption, whereas the sensory system is inter- 

 rupted at no less than three points, and the influence of this factor may not be inconsiderable. 



The interrupted anatomical arrangement just alluded to may also have some bearing on 

 confusing results regarding restitution of function, for, whereas in the case of the anthropoid 

 ape and other animals evidently more so than in the human subject recovery of power after 

 lesions of the motor area occurs to a remarkable extent, the recovery from an experimentally- 

 produced disorder of sensation is even more rapid and complete. Of the many neurological 

 problems awaiting solution this question of the restitution of function is one of the most 

 interesting and, in spite of the various hypotheses which have been advanced to meet it, most 

 unintelligible. 



REMARKS ON THE VARIETIES OF COMMON SENSATION 2 . 



Up to the present I have referred to the postcentral area, either as a terminus for the 

 main tract of common sensory fibres, as a centre for common sensation or as the part of the 

 brain surface on which common sensory impressions impinge, and I have purposely employed 

 these loose and general terms to avoid complications and to keep within the bounds of histological 

 license. Now, however, I must directly trespass for a short time 011 clinical and physiological 



1 It is weak to contend that the occasional occurrence of hemiopia, loss of hearing, taste, or smell, in combina- 

 tion with hemianaesthesia, is important, because it can be proved that in all the cases so complicated the lesion has 

 been either very extensive or deep-seated, and, to my mind, these are true instances of dynamic influence and of the 

 very class of case which we have to guard against and eliminate in forming judgment on points in localisation. 



- I may be taken to task for including the tactile sense in " common sensation," but I understand that " common 

 sensation," although a loose term, is the equivalent of the German " Korpersensibilitat," that it denotes the several 

 cutaneous and other sensory impulses conveyed by the common nerves of the body as distinguished from those passing 

 along special nerves, or parts thereof, and that it has a narrower signification than "general sensibility," the faculty by 

 which we are informed in a general way of corporeal changes in condition and circumstance. 



