152 HUMAN PHYSIOLOGY 



the opposite side, it has been concluded that a few of both 

 sets of fibres decussate in the cord. It will afterwards be 

 shown that the main set of fibres cross the middle line in 

 the medulla (Fig. 83). 



The two kinds of fibres run in different strands or tracts 

 of the cord, and these tracts have been defined by different 

 methods. 



1. Degeneration or Wallerian Method. This depends 

 upon the fact that nerve fibres degenerate when cut off 

 from their cells (p. 86), and that they, generally speaking, 

 conduct in the direction in which they degenerate, although, 

 as has been seen in the fibres of the posterior roots of spinal 

 nerves, this is not always the case. If the spinal cord be cut 

 across, certain tracts of fibres degenerate upwards, others 

 degenerate downwards, while some do not degenerate to any 

 great distance from the point of section. 



Degenerations which reveal these tracts are often produced 

 by diseases and injuries of the cord, and thus the results 

 experimentally produced on animals have been confirmed on 

 the human subject. These degenerations may be demon- 

 strated when recent by Marchi's method of staining, which 

 depends upon the fact that while the white sheath of normal 

 fibres is not stained black when the tissue is placed in a 

 solution of chrome salt with osmic acid, it is so stained when 

 it begins to degenerate (p. 86). When the white sheaths 

 have entirely disappeared the degeneration is best demon- 

 strated by Weigert's method of staining the white sheaths of 

 normal fibres with hsematoxylin, which leaves the degenerated 

 fibres unstained. 



A. Fibres degenerating upwards (see Fig. 78, p. 148). 

 1. The fibres making up the posterior columns which are 

 derived from the posterior roots of the spinal nerves (P.L. 

 and P.M.). A few of these fibres degenerate downwards, 

 because the fibres of the posterior roots bifurcate when they 

 enter the cord (Fig. 40, p. 89), the one division passing right 

 up to the top of the cord, the other passing down for a short 

 distance. The ascending fibres of the posterior columns for 

 the most part end in synapses in the upper end of the cord, 

 in the nucleus gracilis and nucleus cuneatus (Fig. 40, p. 89). 

 From these fresh fibres pass upwards to the cerebrum 



