iv] Postcentral or Seiixory Ana 85 



being so abundant in the radiary zone on the Rolandic face of the gyrus in question, in the 

 adult cortex. Apprised of this band of fibres at the time when the postcentral gyrus was 

 being misted from the motor area, and associating their existence with my knowledge of the 

 peculiar cortical structure of the postcentral gyrus, I was not long in formulating the idea 

 that this gyrus was to be marked as a sensory centre, and I would now say, that if the belief 

 that this tract maturates prior to the motor tract be correct, and if also it can be proved to 

 be the continuation of the "cortical lemniscus," the histological evidence to the effect that the 

 two central gyri subserve different functions will approach completeness. And surely such a 

 separation of function is one which we would look for on a priori anatomical grounds, for 

 when we reflect that, at the level of the spinal cord, the cell stations for the respective links 

 in the sensory and motor neuronic chains are separately situated in the posterior root ganglia 

 and anterior conma, and, that all along the cord, medulla, and pons they pursue distinct paths, 

 and again in the internal capsule occupy different compartments, we naturally hesitate to believe 

 that a similar separate arrangement does not obtain regarding the departure and arrival plat- 

 form to quote Mott's apt simile of these systems of fibres on the surface of the brain. 



EXAMINATION OF THE POSTCENTRAL GYRUS IN CERTAIN DISEASED 



CONDITIONS. 



I have now to record some results which I have obtained from the examination of the 

 cortex of the postcentral gyrus in cases in which there has been some antecedent destructive 

 interference with the main system of sensory neurones at a lower level, for early in this 

 research it occurred to me that such material would prove valuable for the determination of 

 postcentral function, and the special cases which I have selected to meet the purposes of my 

 investigation have been ones of Tabes Dorsalis, of amputation of one or other extremity, and 

 of small lesions cutting the sensory path in its course through the internal capsule. 



The Postcentral Gyrus in Tabes Dorsalis. 



Tabes Dorsalis supplies us with a most exquisite natural destruction, virtually limited in 

 its effects to the main chain of sensory neurones, and I have long been of opinion that no 

 better disease, or purer condition, could be chosen to assist us in the determination of the area 

 in the cerebral cortex which we are to regard as the sensory terminus ; because, practically, we 

 can look upon the tabetic process in the spinal cord as a progressive and eventually comprehensive 

 degeneration, following the stream of physiological conduction and complying with all the postu- 

 lates of the law of Waller; and although, on account of the interposition of a stop in the nuclei 

 of Goll and Burdach, the more obvious histological manifestations of that degeneration cannot 

 be displayed above this level, it is hard to believe that its effects, and especially the complete 

 interruption of physiological impulses which it involves, are not carried over into the succeeding 

 higher neurones, and above all that reactive changes do not occur in the cortical cells cut off 

 from the impact of these stimuli. 



To my mind, the several reasons why the changes to which I allude have not been 

 previously discovered, or even investigated, are that we have not had any clear idea of the 

 residence of these cortical terminals ; our knowledge of the normal histology of the cortex, as 

 regards the distribution of topographic variations in structure, has been imperfect ; our views 

 concerning sensory localisation have been confused by erroneous conceptions regarding the 



