STRUCTURE OF THE SPINAL CORD. 457 



the horizontal fibres next to be mentioned, being intermediate be- 

 tween the latter and the. fibres of the cerebrum. 



The transverse or horizontal fibres occur, 1st, in the lateral and 

 posterior columns adjoining the horns of the gray substance; 2c%, 

 in the white commissure; and, 3c%, at the point of entrance of the 

 roots of the nerves. The first will be described with the gray sub- 

 stance. The fibres in the white commissure come from the anterior 

 columns, and, bending obliquely inwards, cross in front of the gray 

 commissure to the opposite side. The white commissure is thus a 

 decussation of the anterior column, and not a commissure, as gene- 

 rally understood. The decussing fibres measure T ^FO to Wuu f 

 an inch, and decrease as they diverge in the anterior horns of the 

 gray matter. The fibres in the roots of the spinal nerves are con- 

 tained in larger fasciculi, either horizontal, or slightly ascending 

 between the longitudinal fibres to enter the anterior and posterior 

 horns of the gray matter, where we shall again meet them. The 

 fibres do not all communicate with the longitudinal; and in their 

 posterior roots about two-thirds of them measure -j^nnr to y^otr of 

 an inch, and one-third of them T oW to 4 oW f an inch. In the 

 anterior roots about three-fourths of the fibres measure from 2 ^Vtf 

 to lAx f an inch, and one-fourth of them ^oW to isW of an 

 inch. They, however, constantly decrease in size as they proceed 

 through the white matter, and when they enter the gray matter the 

 motor fibres are only $ Q to 3 oW f an inch, while the sensory 

 are but TOITTTO to 42*55 f an inch. 



2. The nerve-fibres of the gray substance of the cord are very 

 numerous, constituting, in any case, one-half of its bulk, or more. 

 They present the same characters as the fibres of the white sub- 

 stance, except that they are not, on the average, more than one-half 

 as thick ( T 5^oo- f an inch). 



As they pass among the nerve-cells of the gray matter, some of 

 the motor fibres have no connection with the processes of the cells ; 

 but continue to run in the anterior horns to the lateral parts of the 

 anterior commissure, and become continuous with the fibres of the 

 latter. Thus some of the motor fibres are connected with the lon- 

 gitudinal fibres of the anterior columns, with 'a total decussation. 

 Many of the motor fibres, however, take no part in this decussation, 

 especially those which enter the anterior horns most externally. 

 These penetrate transversely to various depths (one-half or more), 

 then curve upwards, and finally appear as longitudinal fibres. Thus 



