3G2 SPECIAL HISTOLOGY. 



(Fig. 141, Jc), 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 gray commissure ; the fibres coming from the right 

 anterior column, passing, in a radiating manner and horizontally, to the 

 left anterior horn of the gray substance, and those from the left anterior 

 column passing in like manner to the 7-ufJit anterior horn. The anterior 

 commissure, therefore, represents a decussation, or crossing of the 

 anterior colwnns, 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 0-0012-0-003 of a line, 

 and, as they diverge in the anterior horns, evidently in some degree 

 decrease in diameter. 



The roots of the spinal nerves (Fig. 141, g, w), without any communi- 

 cation with the longitudinal fibres, are continued, in larger fasciculi, from 

 the sulcus lateralis anterior and posterior, either horizontally or slightly 

 ascending between those fibres, in order, all of them, to enter the ante- 

 rior and posterior gray lamina, where we shall again meet them. Their 

 nerve-fibres (in the posterior roots, about |ds measuring 0'004-0*008, 

 and §d 0*0012-0*003 of a line, in the anterior about fths measuring 

 from 0-006-0-011, and ith 0-0025-0 -003 of a line) present, as soon 

 as they have entered the cord, all the characters of the central fibres, 

 the largest, at their commencement, measuring about 0"004-0"006 of 

 a line, in the sensitive, up to 0*008 of a line, in the motor roots. 

 They continue, however, distinctly and constantly decreasing in size, 

 until ultimately, the former, when they enter the gray substance, have 

 a diameter scarcely more than 0*0012-0-0028 of a line, and the latter 

 in like manner one of not more than O'OOl, or in some of 0*006 of a line. 



In the gra?/ substance, the nerve-cells and fibres deserve special 

 consideration. The former present very various forms, all, however, 

 corresponding in one respect, that they are invariably furnished with 

 processes or prolongations, and, for the most part, with many such, 

 which repeatedly branching, ultimately terminate in extremely fine, 

 pale fibrils, like the finest axis-fibres. I distinguish : — 1. The cells of 

 the central gray substance. These (Fig. 142) are 0*004-0*008 of a 

 line, in size, always pale and finely granuUir, with multiple nuclei and 

 branching pale processes, constituting as it would seem the principal 

 bulk of the central gray substance ; but in which, dark, true nerve- 

 fibres also occur although of nearly the finest kind, which can scarcely 

 be recognized as such, and are indeed very few in number, and quite 



