24 -.;. THE SPINAL CORD. 



the part of the fibre thus separated from that centre (secondary degenerations], or they 

 may follow from the prolonged disuse of a nervous tract, especially in young animals, 

 as when a limb has been removed or from some other cause (tertiary degenerations). 

 The degeneration which follows a lesion of part of the nervous centre, and especially 

 section of the spinal cord, is in some tracts above the lesion, in which case the 

 degenerated tract is styled "ascending," in others below ("descending") (see also, 

 p. 27). The place of the degenerated nervous substance is ultimately taken by a 

 non-nervous fibrillar tissue, which, by its difference of behaviour to staining fluids, 

 can easily be distinguished from the surrounding undegenerated white substance 

 (stage of sclerosis). In new-born and young animals, after a longer interval, groups 

 of nerve-cells are affected by the degenerative processes, and the cells may even 

 eventually disappear altogether (v. Gudden). When this is the case it may be 

 assumed that they are in connection with the fibres which have been cut ; these 

 being probably the axis-cylinder processes of the cells. 



Thus in a rabbit in which, immediately after birth the sciatic trunk is resected, when the 

 animal is full-grown the corresponding anterior cornual cells are found to have become 

 atrophied, and similar results are obtained with motor nerves generally, both spinal and 

 cranial (v. Gudden, Forel, Ganger, Mayser). 



Tracts of the antero-lateral column. Several tracts can by these methods 

 be traced in the antero-lateral column, not only along a great part of the spinal cord, 

 but into or from certain parts of the encephalon. The long tracts in this column 

 are the direct and crossed pyramidal (descending), the antero-lateral or ventro-lateral 

 ascending, the antero-lateral or ventro-lateral descending, and the dor so-lateral 

 ascending (or direct cerebellar). 



Descending tracts in the antero-lateral column. The pyramidal tract 

 is directly traceable down from the opposite pyramid of the medulla oblongata, and 

 ultimately from the cerebral cortex (Rolandic region). The greater number of the 

 fibres which compose the pyramid cross at the upper limit of the spinal cord, down 

 which they pass in the posterior part of the lateral column as a compact bundle of 

 fibres occupying in transverse section a somewhat triangular area, which lies in the 

 angle between the posterior horn and the outer surface of the cord, but is in most 

 parts separated from both by fibres belonging to other systems. This lai&ral or 

 crossed part of the pyramidal tract (figs. 14 and 24), can be traced as far as the third or 

 fourth pair of sacral nerves, becoming gradually smaller below and approaching the 

 surface of the cord. 



A few fibres of the pyramidal tract are found scattered in other parts of the 

 antero-lateral column. 



Some of the fibres of the pyramids of the medulla oblongata do not decussate at the 

 upper limit of the cord. These pass down close to the anterior median fissure, forming 

 the anterior or direct portion of the pyramidal tract (fig. 14) (column of Turck), which 

 gradually diminishes as it is traced downwards, and usually ceases altogether at 

 about the middle of the dorsal region of the cord. It is probable that the decus- 

 sation of these anterior pyramidal tracts goes on along their whole course, their 

 fibres passing through the anterior commissure and through the grey matter of the 

 opposite side to reach the lateral pyramidal tract on the other side of the cord. 



There is much variation in the development of the anterior pyramidal tracts in different 

 individuals. In some they are so well marked as to form a visible prominence on the surface 

 of the cord close to the anterior median fissure and separated from the rest of the anterior 

 column by a groove, the anterior Intermediate sulctis of Rauber. In others they are quite small, 

 or may even fail altogether. In this case it may be assumed that the decussation of the pyra- 

 mids, which is known to be subject to considerable variation, has been more complete than 

 usual. In other cases again the anterior pyramidal tracts may be unsymmetrical, being 



