CH. 



DESCENDING DEGENERATION. 



613 



for instance those from the upper limbs and neck, pursue their 

 course in the inner part of the column of Burdach. 



The preceding figure (fig. 454) shows the degeneration in a 

 section of the spinal cord, after the division of a number of 

 nerve-roots on one side. The microscopic section is taken high 

 up, so that all the degenerated fibres have passed into the 

 column of Goll on the same side ; the inner set ( i ) are shaded 

 differently from the outer set (2), indicating that those nearest 

 the middle line come from the lowest nerve-roots. 



We may pass from this to consider the tracts of degeneration 

 that occur when the spinal cord is cut right across in the thoracic 

 region. Some tracts will be found degenerated in the piece of 

 cord below the lesion ; these consist of nerve-fibres that are 



Crossed 

 Pyramidal 



Antero-tateral descending" 



Direct Pyramidal 

 Fig. 455. Descending tracts of degeneration. 



connected with the nerve-cells in the brain ; they are called the 

 pyramidal tracts. Other tracts are found degenerated in the 

 piece of cord above the lesion ; these consist of nerve-fibres that 

 are connected with the nerve-cells of the spinal ganglia, or with 

 the cells of the spinal cord itself below the lesion and are passing 

 upwards. 



The tracts which degenerate downwards are the motor tracts ; 

 the tracts that degenerate upwards are the sensory tracts. 



If the animal is killed a few weeks after the operation, its cord 

 removed, and microscopic sections of it made and stained in an 

 appropriate manner, the ascending tracts will be found degene- 

 rated in the piece of cord above the lesion ; the descending tracts 

 degenerated in the piece of cord below the lesion. These are 

 shown in figs. 455 and 456. 



Tracts of descending degeneration (fig. 455). 



(i.) The crossed pyramidal tract. This tract is situated in the 



