838 THE CENTRAL NERVOUS SYSTEM 



When the spinal cord is divided, and the animal allowed to survive 

 for a time, certain tracts are picked out by the degeneration of their 

 fibres, although in every degenerated tract some fibres remain un- 

 affected. We may distinguish the tracts that degenerate above 

 the lesion (ascending degeneration) from those that degenerate 

 below the lesion (descending degeneration). 



Ascending Tracts. Above the lesion degeneration is found both 

 in the posterior and the antero-lateral columns. Immediately 

 above the section nearly the whole of the posterior column is in- 

 volved. Higher up the degeneration clears away from Burdach's 

 tract, and, shifting inwards, comes to occupy a position in the 

 column of Goll. In the antero-lateral column two degenerated 

 regions are seen, both at the surface of the cord, one a compact, 

 sickle-shaped area extending forwards from the neighbourhood of 

 the line of entrance of the posterior roots, and the other an area of 

 scattered degeneration, embracing many intact fibres, and complet- 

 ing the outer boundary of the column almost to the anterior median 

 fissure. The compact area is called the dorsal or direct cerebellar 

 tract, or tract ofFlechsig (or the fasciculus cerebello-spinalis},i~\\Q diffuse 

 area the antero-lateral ascending tract, or tract of Cowers, or ventral 

 cerebellar tract (or the fasciculus antero -later alls superficialis).* The 

 dorsal cerebellar tract is distinguished by the large size of its fibres. 

 It is only distinct in the dorsal and cervical regions of the cord. The 

 tract of Lissauer, or posterior marginal zone, is another small ascend- 

 ing tract at the outer side of the tip of the posterior horn. It is 

 made up of fine fibres from the posterior roots which soon pass into 

 the posterior column. 



Descending Tracts. When the cord is divided, say, in the upper 

 dorsal or cervical region, the following tracts degenerate below the 

 lesion : 



(1) A small group of fibres close to the antero-median fissure, 

 which has received the name of the direct pyramidal tract- pyramidal 

 because higher up in the medulla oblongata it forms part of the 

 pyramid ; direct, because it does not cross over at the decussation of 

 the pyramids, but continues down on the same side. In the stan- 

 dard anatomical nomenclature it is termed the fasciculus cerebro- 

 spinalis anterior. The direct pyramidal tract is only present in man 

 and the higher apes. 



(2) A tract of degenerated fibres in the posterior part of the 

 lateral column. This is the lateral or crossed pyramidal tract (or the 

 fasciculus cerebro-spinalis later alis) , and is much larger than the direct . 

 In the medulla it also lies within the pyramid, but, unlike the direct 

 pyramidal tract, it crosses to the opposite side of the cord at the 

 decussation. The pyramidal tracts are also called cortico-spinal to 

 indicate their origin and termination. 



* Some writers employ the terms dorsal and ventral spino-cerebellar 

 tracts. 



