THE NERVOUS SYSTEM. 409 



will be afterwards shown, they successively pass inwards 

 towards the grey substance, presenting the general characters 

 of the central nerve-fibres; that is to say, the delicacy of 

 sheath, disposition to the formation of varicosities, and to the 

 breaking up into separate fragments, which are constituted 

 either of all the elementary parts of the nerve-tubes, or consist 

 of nothing more than an axis-fibre, or of the medullary- 

 sheath. Their diameter amounts to 0-0012 — 0-004-8'", on the 

 average 0*00:2 — 0-003'", and, in one and the same fibre is, 

 evidently, always nearly the same, since, in the white substance, 

 neither divisions nor any other kind of alteration in diameter 

 of the fibres are found to exist. The transverse fibres 

 occur : — 1 . in those portions of the lateral and posterior 

 columns which adjoin the horns of the grey substance, and the 

 description of which will be given afterwards with that of the 

 grey substance; 2. in the white commissure; and, 3. at the 

 points of entrance of the roots of the nerves. The white, or 

 anterior commissure (fig. 141, k) is not a commissure in the 

 common sense of the word. It is formed by those nerve-fibres 

 of the anterior columns, which are, in succession, the most 

 deeply placed, and which bending obliquely inwards, cross in 

 front of the grey commissure; the fibres coming from the 

 right anterior column, passing, in a radiating manner and hori- 

 zontally, to the left anterior horn of the grey substance, and 

 those from the left anterior column passing in like manner to 

 the right anterior horn. The anterior commissure, therefore, 

 represents a decussation, or crossing of the anterior columns, 

 and would be better designated as such. It varies both in 

 thickness and breadth; it is thickest in the region of the two 

 enlargements, and is least so in the middle of the dorsal 

 portion of the cord. Its breadth is regulated pretty nearly by 

 that of the cord, and of the bottom of the anterior fissure ; 

 being greatest in the cervical enlargement, and from this point 

 decreasing pretty uniformly in both directions. The decussating 

 fibres measure 0-0012 — 0003'", and, as they diverge in the 

 anterior horns, evidently in some degree decrease in diameter. 

 The roots of the spinal-nerves (fig. 1-1-1, g, w), without any 

 communication with the longitudinal fibres, are continued, in 

 larger fasciculi, from the sulcus lateralis anterior and posterior, 

 either horizontally or slightly ascending between those fibres, 



