CHAP, i.] THE SPINAL CORD. 875 



iiig tract and the crossed pyramidal tract on the outside and the 

 grey matter on the inside. From this area of white matter we 

 may put on one side at present the external posterior column 

 because, as we shall see, this column is largely composed of the 

 fibres of the posterior root which pass through this column, 

 especially through the lateral part of it near the grey matter, on 

 their way to their ultimate destination ; hence the alternative 

 name of posterior root-zone. We may similarly leave for the 

 present the small zone of white matter composed of very fine 

 fibres known as the posterior marginal zone or Lissauer's zone (Fig. 

 100, L.), lying dorsal to the tip of the posterior horn and in the 

 lower regions reaching to the outside of the cord ; for this too 

 belongs to the fibres of the posterior root. Leaving these parts 

 out of consideration we may say as regards the rest of the white 

 matter, that the present state of our knowledge will not allow us 

 to divide it into special tracts. All this area is largely composed 

 of fibres which do not undergo either ascending or descending 

 degeneration as the result of section, injury or disease. It has 

 been suggested that these fibres either have no trophic centre at 

 all or have double ones, one above and one below, on either of 

 which they can in case of need lean ; so that when the fibre is 

 divided at any level, the upper portion is still nourished from 

 some centre above, and the lower from some centre below. At 

 all events, whether this be the true explanation or no, the fibres 

 in this part of the white matter cannot be differentiated into 

 tracts by a study of their degeneration. Fibres of this kind, 

 which we can speak of neither as ascending nor as descending, 

 also occur in the external posterior column mingled with the 

 fibres of the posterior root. And we may repeat the caution, 

 that even in the several ascending and descending tracts just 

 described, especially in those which we spoke of as less distinct or 

 as more diffuse, many fibres are present which undergo neither 

 ascending nor descending degeneration. 



568. It may be as well perhaps to insist here once more, 

 that when these several tracts or the fibres running in the tracts 

 are spoken of as ascending or descending, what is meant is that 

 the degeneration takes place above the section or seat of injury or 

 disease in the one case, and takes place below in the other. It 

 has been supposed by many that the nervous impulses which 

 these fibres severally carry, travel in the same direction as that 

 taken by the degeneration, that the ascending tracts carry impulses 

 from below upward, that is to say, carry impulses which arising 

 from peripheral organs pass to various parts of the spinal cord or 

 of the brain, that they are, in other words, channels of afferent 

 impulses, and that conversely the descending tracts carry efferent 

 impulses. To this view is often added as a corollary, that the 

 tracts which do not degenerate at all carry impulses both ways, 

 and hence cannot be considered as either afferent or efferent 



