LOCOMOTOE ATAXY. 223 



lumbo-sacral region. It is doubtful if any fibres are contributed 

 to this column by the posterior roots in the thoracic region from 

 the third to the twelfth inclusive. The corresponding ascending 

 fibres from the posterior roots of the cervical enlargement are 

 said to continue their ascent in Burdach's column, reaching the 

 nucleus of that column in the medulla. 



All these sets of fibres may degenerate in tabes, and in addition 

 the process in some cases extends to the cells of Clarke's column, 

 and to the direct cerebellar tracts derived from these. 



Attempts have been made to correlate the different sensory 

 symptoms of the disease with the different fibre groups. Thus 

 the loss of muscle tone and of the tendon reflexes is referred to 

 the interruption of those fibres which pass directly to the anterior 

 cornual motor cells, and so complete the local reflex arc. It is 

 also conjectured that the long ascending fibres of the postero- 

 internal columns subserve the muscle sense, that the fine fibres 

 which enter the grey matter through Lissauer's tract are fibres 

 of common sensation, and that the fibres which establish a con- 

 nexion with the cells of Clarke's column convey impulses to the 

 cerebellum, and so form part of the mechanism of equilibration. 



There are certain tracts of fibres in the posterior columns of 

 the cord, which originate from the cells of the grey matter of the 

 posterior cornua, and are hence termed endogenous. These tracts 

 are commisural in function, and do not consist of fibres derived 

 from the posterior roots. They either escape entirely in tabes or 

 are affected very late in the disease. The tracts in question 

 are: (1), the comma tract which lies in the middle third of the 

 postero-external column in the dorsal region ; (2), the cornu- 

 commisural zone, which is immediately adjacent to the pos- 

 terior commisure in the lumbar and sacral regions ; (3), the oval 

 area of Flechsig, which abuts on the posterior fissure in the 

 lumbar region, and is continuous in the sacral region with (4) 

 the sacral triangle of Gombault and Phillipe ; (5), the septo- 

 marginal tract, which is continuous with the oval area of 

 Flechsig, and can be traced upwards as high as the eleventh 

 dorsal segment. 



