Secondary Degeneration following Unilateral Lesions etc. 321 



fibres coming' off from the, left pyramid, crossing the median raphe 

 and then intermingling with bundles of normal fibres from the oppos- 

 ite P3a'amid; they disappear amongst the internal ai'cuate fibres on 

 the right side close to the middle line. No homolateral fibres are 

 visible at this level. About the middle of the decussation the pyra- 

 mid has become much reduced in size, and large bundles of degener- 

 ate fibres cross the middle line to interlace with similar bundles from 

 the normal (right) pyramid. Numerous large fasciculi of degenerated 

 fibres cut transversely are situated within the grey matter between 

 the base of the substantia gelatinosa of Rolando and the lateral nuc- 

 leus on the right side. These consist of fibres Avhich have crossed 

 at a higher level and are now descending to reach the lateral column 

 of the spinal cord, there to form the right crossed pyramidal tract. 

 A very few homolateral fibres are seen passing towards the crossed 

 pyramidal tract of the same side; these are more abundant in sec- 

 tions through the lower levels of the decussation (figs. 12 and 13. 

 pi. XVII). At the level of the second cervical segment of the spinal 

 cord the ratio of crossed to uncrossed fibres is about 1 00 : 1 . 



In sections through the 6*^ cervical segment of the spinal cord 

 the right crossed pyramidal tract appears as a wedge-shaped area 

 crowded with degenerated fibres, quite unlike the rounded area in the 

 cat and dog, and absolutely, and relatively to the rest of the lateral 

 column, much larger. The direct cerebellar tract lying between it 

 and the margin of the section contains here and there a degenerated 

 fibre but it is practically free from degeneration (fig. 14. pi. XVII). From 

 the inner border of the right crossed pyramidal tract a few fibres, 

 cut obliquely, can be seen passing towards the base of the posterior 

 horn, and a considerable amount of fine degeneration can be made out 

 in the grey matter at the base of the posterior horn, while none is 

 visible in any other part of the grey matter. A few homolateral 

 fibres are present in the lateral column of the left side. About the 

 mid-dorsal region the right crossed pyramidal tract, now considerably 

 reduced in size, extends nearly to the margin of the lateral column, the 

 direct cerebellar tract having almost disappeared at this level. No fibres 

 are seen entering the grey matter (fig. 15. pi. XVII). There are fewer 



Internationale Monatsschrift für Anat. u. Phys. XIX. 21 



