THE ROOTS OF THE PERIPHERAL NERVES, ETC. 



159 



gray matter, of numerous cells with long, slender processes. The 

 body of the cell is usually so small that the processes have the 

 appearance of springing from the nucleus. Occasionally, how- 

 ever, it is larger. These cells are called the spider-cells, or cells 

 of Betters. (See Fig. 95, at left). 



The spinal cord is, as a glance at a transverse section will 

 show, divided into several 

 columns by the entering nerve- 

 roots and the longitudinal fis- 



O 



sims. Internal to the spinal 

 nerve-roots are situated the 

 anterior or posterior columns, 

 as the case may be, and exter- 

 nal to them the lateral columns. 

 The study of embryology 

 and the investigation of cer- 



O 



tain diseases of the spinal cord 

 show that the anterior, lateral, 

 and posterior columns are not 

 uniform masses of fibres of 

 equal importance, as might 

 appear from a section through 

 the adult organ, but that they 

 are composed of several dif- 

 ferent divisions. 



You remember the pyra- 

 midal trnct wllOSP rniirSP WP Diagram of descending degeneration in the 

 jl t pyramid, with a lesion in the left internal 



traced from the cortex of the 



motor zone through the capsule and the crusta to the ventral 



part of the pons. 



Let us ascertain in what part of the spinal cord this tract 

 is situated. It is not difficult to discover this. If a destructive 

 disease involves any part of this extensive tract the nerve-fibres 

 disappear, and are replaced by connective tissue. This process, 

 which is called secondary degeneration, continues downward 



Fro. 96. 



