80 Anatomical Considerations regarding Function [CHAP. 



information which we can command concerning the localisation of " common sensation," the in- 

 vestigation of this subject is beset with unusual difficulties. I shall mention later that the 

 knowledge which we have obtained from an accumulation of clinical observations on the human 

 subject needs revision, and that as far as experiment on lower animals goes, the information 

 gained is so inadequate that most physiologists have temporarily abandoned hope of further 

 gain from this method of investigation. To my mind, however, this unsettled state of affairs 

 is in large measure attributable to two causes : one is the long-standing and firmly rooted 

 but erroneous belief, that the two central gyri are both motor in function, and the second 

 is that too little regard has been paid to a thorough preliminary investigation of the anato- 

 mical pathway and cortical connections of the main sensory tract. The first of these causes 

 has already been removed, and I shoiild like now to draw attention to some points in anatomy 

 and histology, normal and morbid, bearing on this subject, which may be of service to the 

 experimenter and clinician in the execution of future observations, and which in my opinion 

 favour the assumption that the cortex of the postcentral gyrus is as strictly a sensory centre 

 as is the visual or auditory area. 



ANATOMICAL AND HISTOLOGICAL CONSIDERATIONS. 



From studies of secondary degenerations and retrograde atrophies and from observations on 

 the developing brain, it is now definitely settled that the sensory tracts of fibres in the spinal 

 cord, after being interrupted in the nuclei of Goll and Burdach, are continued onwards in 

 the medial lemniscus 1 ; and, that just as the motor fibres, or at any rate a major proportion 

 thereof, decussate at the lower end of the medulla, so also these sensory fibres cross over 

 in the decussatio lemniscorum. The further course of these fibres has been studied in both 

 man and the lower animals by von Gudden, Flechsig and Hcisel, Monakow, Mahaim, Mott, 

 Tschermak and many others, and it is now believed that save minor connections, for instance 

 with the superior colliculus, which need not be considered here, the bulk of the fibres gain 

 and are interrupted in the optic thalamus. This conclusion, however, has not been reached 

 without a considerable amount of discussion, for certain writers maintained for a long time 

 that most of the fibres composing the medial lemniscus proceeded without interruption through 

 the internal capsule and out in the corona radiata to the cortex cerebri ; and even though 

 it is now admitted that this was a false belief, so far as the bulk of the fibres is concerned, 

 it is still considered probable that a certain number do follow this uninterrupted course. But 

 to us this is a point of minor interest compared with a consideration of the course and 

 destination of the next link in the system, that extending from the region of the thalamus 

 to the cortical surface. 



This collection of neurones is best known as the " cortical lemniscus " and is usually 

 associated with the name of von Monakow, because that observer was not only the first to 

 describe it, but has also supplied us with more information concerning it than any other writer. 

 The term owes its introduction to von Monakow's discovery that ablation of that portion of 

 the parietal lobe in the cat which corresponds to Munk's zone F, was followed by changes 



1 This is not to be confused with two other tracts of fibres bearing the name lemniscus, viz., the lateral 

 lemniscus, which has been proved to be a pathway for centripetal auditory impulses ; and the medial accessory 

 lemniscus, which is to be distinguished because it has been proved that its fibres are late in acquiring their myelinic 

 sheath and also degenerate iu a descending direction. 



