iv] 1'onfci'iifru/ or Sensory Area 83 



Darkschewitch ten years ago ami it lias been continued by Bregmann, Klippel and Durante, 

 van (iehuchten, and many others since, that the negative proposition contained in the law 

 of Waller, nainelv, that a nerve fibre separated from its trophic centre does not degenerate 

 in its proximal end, is a false one: and, in fact, this retrograde or indirect Wallerian degene- 

 ration, as it is called, is histologically indistinguishable from the true and originally described 

 form, but differs pathologically in being more tardy in making its appearance; for whereas the 

 true Wallerian degeneration manifests itself six, seven, or eight days after the lesion, the 

 indirect form is not evident until about the twentieth day. Therefore, since much valuable 

 work has been done by this method and it seems thoroughly reliable for tracing tracts of 

 fibres, it only remains for ns to try to work out the course of the "cortical lemniscus" by 

 st .inlying the degenerations running in association with lesions of the postcentral gyrus ; for 

 this, lesions produced experimentally in lower animals, preferably the ape, or natural lesions 

 in the human brain would be suitable, but as it seems impossible to cause destruction 

 of a large portion of the postcentral, or any other gyrus, without injury to neighbouring 

 convolutions and other tracts of fibres than the one it is desired to investigate, I would 

 suggest that limited lesions, if possible confined to the cortex only, be made or chosen for 

 the purpose. 



Points in the Development of the Sensory Tracts. 



As Flechsig was able to demonstrate with such clearness and certainty, that the fibres in 

 the posterior columns of the spinal cord standing in connection with the posterior roots are 

 distinguishable from all the other long systems of fibres inasmuch as they acquire their 

 myelinic investment at a remarkably early period, it might be supposed that the course of the 

 upper segments of the same neuronic chain could for a like reason be readily picked out ; but 

 although this may be said to apply to the bulbo-thalamic segment, it does not appear to hold 

 in the case of the thalamo-cortical. There are, however, several reasons why the pursuit of this 

 upper portion of the sensory tract should be attended with difficulty, and perhaps the most 

 important is, that in this particular region, and certainly in the vicinity of the thalamus and 

 internal capsule, the arrangement of structures is of an exceedingly complex nature ; and apart 

 from this serious impediment we have to bear in mind that with an increase in the size of the 

 section the troubles in regard to orientation are proportionately amplified. What I mean to 

 say is that while it may be a simple matter to follow parallel-lying tracts of fibres in a small 

 area like that covered by a transverse section of the spinal cord, it is otherwise when we have 

 to explore complete sections of a cerebral hemisphere, wherein the direction of the fibre bundles 

 is one of infinite disorder; add to this the fact that developmental workers have been exposed 

 to the liability of being led astray by having to take notice of the belief that both the central 

 gyri were motor in function, and the shortcomings in this connection of which we may have 

 to complain will be understood. 



Now as Flechsig stands preeminent among neurologists who have devoted their attention 

 to embryological research, it is necessary that we should first enquire into his findings and 

 conclusions concerning the tract of immediate interest, the "cortical lemniscus." In his great 

 work, he tells us that the centripetal or sensory fibres are found more or less ''en masse" in 

 the posterior portion of the pars occipitalis of the internal capsule, and that they are divisible 

 into three sets, according to the different periods at which they maturate. The first set appears 

 at about the beginning of the ninth month of foetal life, and is said to spring from the nucleus 



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