THE NERVOUS SYSTEM. 861 



a preponderance of smaller nerve-cells the substantia gelatinosa of 

 Rolando. Of the roots of the spinal nerves, the anterior penetrate be- 

 tween the anterior and lateral columns, di- 

 rectly to the anterior horns, and the poste- 

 rior are lost between the lateral and pos- 

 terior columns, passing through the substan- 

 tia gelatinosa into the posterior laminae or 

 horns. 



With respect to the. intimate structure of 

 the spinal cord, we have to distinguish in the 

 white substance : 1, horizontal ; and 2, lon- 

 gitudinal fibres. The latter, in all situations, 

 except in the anterior commissure, are in 

 great part altogether unmixed with horizon- 

 tal fibres, and everywhere, both superficially 



and deeply, run parallel with each other, but they are never in- 

 terlaced, nor do they ever constitute smaller fasciculi. The number 

 diminishes from above downwards, because, as will be afterwards 

 shown, they successively pass inwards towards the gray substance, 

 presenting the general characters of the central nerve-fibres ; that 

 is to say, the delicacy of sheath, disposition to the formation of vari- 

 cosities, 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 0-0012-0-0048, on the average 0-002-0-003 

 of a linej 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 pos- 

 terior columns which adjoin the horns of the gray substance, and the 

 description of which will be given afterwards with that of the gray 

 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. Transverse section through the spinal cord in the superior lumbar region, mag- 

 nified about 30 diameters, half diagrammatic : a, anterior column ; 6, lateral columns, motor 

 portion; c, lateral columns, sensitive portion; d, posterior columns; e, anterior longitudinal 

 fissure; /, posterior longitudinal fissure ; g, motor roots ; A, their internal fasciculus; i, their 

 external fasciculus ; k, decussation of the anterior columns in the anterior commissure ; I, 

 gray fibres of the lateral columns passing into the anterior gray commissure; w, gray cen- 

 tral nucleus, here internally with two groups of somewhat darker cells ; , posterior gray 

 commissure, with a vessel cut across; o, fibres of the posterior column passing into the gray 

 commissure; p, fibres of the sensitive roots going off to the lateral columns; <?, similar fibres 

 entering the posterior column ; r, longitudinal fasciculi of fibres passing into the sensitive 

 roots; s, substantia gelatinosa, with traversing bundles of the sensitive roots, w ; t, sensitive 

 radical fibres running horizontally forwards to the gray commissure ; M, large cells of the 

 anterior cornua (the puricta), inner group ; v, the same, outer group. 



