chap, xvi.] THE SPINAL CORD. 137 



fibres, simple axis cylinders of various sizes, and primi- 

 tive nerve-fibrillse. 



The medullated nerve-fibres run a more or less 

 horizontal course, and they belong to different 

 sources : 



(1) Medullated nerve-fibres connected directly 

 i.e., by the axis cylinder process (see below) with 

 ganglion cells of tlie anterior horns ; they leave the 

 anterior horns by the septa and septula, and they 

 form the anterior nerve roots. 



(2) Medullated nerve-fibres which form the an- 

 terior white commissure ; they are traceable from the 

 anterior column of one side into the grey matter of 

 the anterior horns of the opposite side, as has been 

 mentioned above; some of these, at any rate, are 

 distinctly and directly traceable to ganglion cells. 



(3) Medullated nerve-fibres derived indirectly 

 from the median bundle of the posterior nerve root 

 i.e., coming out of the cuneiform fasciculus of the 

 posterior column and medullated nerve-fibres derived 

 directly from the lateral bundle of the posterior nerve- 

 root. Both these sets of nerve-fibres can be traced 

 for longer or shorter distances in the grey matter of 

 the posterior horns ; on their way they undergo 

 numerous divisions into very fine medullated fibres. 



(4) Medullated nerve-fibres passing from the grey 

 matter into the white matter of the lateral column. Some 

 of these are nerve-fibres that pass simply through the 

 grey matter of the anterior horns from an anterior 

 nerve root ; others are derived directly from ganglion 

 cells forming the columns of Clarke in the dorsal 

 region (see below). But the majority are derived from 

 that part of the grey matter intermediate between the 

 anterior and posterior horn. 



177. The simple axis cylinders are found very 

 numerously in the grey matter of all parts; they 

 are of various sizes, and run in all directions, many 



