SPIXAL NERVES 741 



clearly undcrstuod that these tracts or columns are onl}- recognisable during their 

 development or when affected with disease or degeneration. 



The following tracts are recognised in the posterior colunms: — 



Ascendino- I '^'^^^ postero-lateral, or Burdach's column. 

 ® 1 The i^ostero-mesial, or GoU's colunm. 



Descending | The comma-shaped tract. 



(fig. 440 A). 



The two ascending columns are not distinguishable from each other below the 

 mid-dorsal region, but above that level they are partially separated by a sejjtum of 

 neuroglia called the postero-lateral septum. The fibres of Goll's column are small 

 and those of Burdach's column are large, but both are derived from the posterior 

 nerve-roots, and as they pass upwards they give off collaterals which enter the grey 

 matter. 



Home of the fibres of the posterior ascending cohmms terminate in arborisations 

 round the cells of the grey matter of the cord, but others pass U})wards to the 

 medulla ol)longata, where the fibres of Goll's column ramify round the cells of the 

 nucleus gracilis, and the fibres of Burdach's column arborise round the cells of 

 the nucleus cuneatus. 



The descending or comma-shaped tract is small, often ill-defined, and it 

 lies in the midst of the postero-lateral ascending tract. It consists of fibres from 

 the posterior roots which are passing downwards to lower segments of the cord. 



The tracts which have been defined in the antero-lateral colunm are the 

 following: — 



{Crossed pyramidal. 

 Direct pyramidal. 

 Antero-lateral descending cerebellar. 



■ (fig. 440 A). 



!Dorso-lateral, or direct cerebellar. 

 Antero-lateral ascending cerebellar. 

 Lissauer's column, or the marginal bundle. 

 Antero-lateral ground Ijundle. 



The descending tracts of the antero-lateral column. — The crossed 

 pyramidal tract is a somewliat triangular Inmdle of nerve-fibres lying in tlic 

 dorsal part of the antero-lateral column, in front of and somcAvhat external to the 

 posterior cornu. It is separated from the surface, except in the lower pait of its 

 extent, by the direct cerebellar tract (fig. 440a). Its fibres are the axons of nerve- 

 cells in the cortex of the opposite side of the brain; they pass downwards througli 

 the corona radiata, the internal capsule, the crura cereliri, the pyramidal l)imdles 

 <^f the anterior pyramid of the medulla to the decussation of the ])vramids; there 

 they cross the middk' line and continue their descent in the o])]iosite antero-lateral 

 column; finally they leave the antero-lateral colunm and pass into the substance of 

 the anterior cornu, where thev terminate in ramifications round the motor cells 

 (fig. 440a). 



The direct pyramidal tract is a small quadrilateral bundle of nerve-fibres 

 situated at the side of the anterior fissure. It cannot be traced downwards beyond 

 the mid-dorsal region. Its fibres are the axons of the nerve-cells of the brain 

 cortex of the same side, which have taken the course descril)ed in connection Avith 

 the crossed pyranndal fibres as far as the medulla; then, instead of crossing to the 

 opposite side, they have descended in tlie anterior column of the same side; ulti- 

 mately, however, they reach the o])})osite side by passing through the anterior 

 white commissure, and they terminate in ramifications round the motor cells of the 

 anterior cornu. 



The descending antero-lateral column is situated peripherally. It extends 

 forwards from the crossed pyramidal and direct cerebellar tracts to the base of the 

 direct ]iyramidal tract, but it is broader dorsally than ventrally, and its fibres nro 

 more or less mixed with those of the antero-lateral ascending cerebellar tract 



