STRUCTURE OF THE SPINAL CORD 



187 



The grey matter, besides neuroglia, consists of an interlacement of 

 nerve-fibres and of the branching processes of the nerve-cells which are 

 embedded in it. 



FIG. 222. A SMALL PORTION OF A TRANSVERSE SECTION OF THE HUMAN SPINAL CORD 



IN THE REGION OF THE LATERAL COLUMN, TO SHOW THE SUPERFICIAL NEUROGLIA. 



a, a, superficial neuroglia ; b, b, transverse section of part of the lateral column of the cord, in 

 which the dark points are the axis-cylinders, and the clear areas the medullary substance 

 of the nerve-fibres. The superficial neuroglia is seen to exhibit the appearance of a fine net- 

 work in which numerous nuclei and one or two corpora amylacea are embedded, and to 

 extend inwards among the nerve-fibres. 



Disposition of the nerve-fibres of the white columns in tracts. 

 The nerve-fibres vary in size in different parts of the white columns. 

 They are smallest in the posterior columns, especially in that part 

 of the posterior column which is next the posterior median fissure 

 (posterior median column}, and largest in the posterior part of the 

 lateral column, but also of considerable size in the anterior column. 

 This difference corresponds in some measure with the functions of the 

 nerve-fibres in those parts respectively. Thus the posterior part of 

 the lateral column which is characterised by the large size of its nerve- 

 fibres constitutes the tract along which voluntary motor impressions 

 pass down the spinal cord from the opposite side of the brain, after 

 having crossed at the pyramids of the medulla oblongata (crossed 

 pyramidal tract, fig. 223, c.p.t.). The large fibres which lie in the 

 anterior columns next to the anterior median fissure, and the upper 

 part of the cord, belong to a portion of the same tract which has not 

 undergone decussation (direct pyramidal tract, d.p.t.}. The small 

 fibres of the posterior median column belong to a tract which is only 

 distinct above the middle of the dorsal region of the cord, and is known 

 as the tract of Goll (fig. 223, g). The rest of the posterior column is 

 termed the cuneate fasciculus, and is chiefly composed of the fibres 

 of the posterior nerve-roots which run for a short distance in it before 

 entering the grey matter. In the lateral column there is another tract 

 which is distinct in the cervical and dorsal regions. This lies external 



