408 SPECIAL HISTOLOGY. 



(subst. grisea centralis), and of nerve-fibres running trans- 

 versely, continued beyond the nucleus, before and behind it — 



the grey, or posterior commissures. 

 Of the laminse, in a transverse sec- 

 tion also termed horns, the anterior 

 are thicker, and shorter, of a uni- 

 form grey colour, composed of larger 

 and smaller nerve-cells, and of deli- 

 cate nerve-fibres of medium fineness; 

 the posterior, longer and thinner, 

 are constituted at their roots like 

 the former, only most usually of 

 smaller cells ; but at the free edge 

 are invested with a more transparent 

 layer, containing a preponderance of smaller nerve-cells — the 

 substantia yelatinosa of Rolando, Of the roots of the spinal- 

 nerves, the anterior peuetrate between the anterior and lateral 

 columns, directly to the anterior horns, and the posterior are 

 lost between the lateral and posterior columns, passing through 

 the substantia yelatinosa 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. longitudinal fibres. The latter, in all situations, ex- 

 cept in the anterior commissure, are in great part altogether 

 unmixed with horizontal fibres, and everywhere, both super- 

 ficially and deeply, run parallel with each other, but they are 

 never interlaced, nor do thev ever constitute smaller fasciculi. 

 The number diminishes from above downwards, because, as 



Fig. 141. Transverse section through the spinal cord in the superior lumbar 

 region, magnified about 30 diam., half diagrammatic : a, anterior column ; b, lateral 

 columns, motor portion; c, lateral columns, sensitive portion; d, posterior columns; 

 e, anterior longitudinal fissure ; f, posterior longitudinal fissure ; g, motor roots ; 

 //, their internal fasciculus ; i, their external fasciculus ; k, decussation of the anterior 

 columns in the anterior commissure ; I, grey fibres of the lateral columns passing into 

 the anterior grey commissure; m, grey central nucleus, here internally with two 

 groups of somewhat darker cells ; n, posterior grey commissure, with a vessel cut 

 across ; o, fibres of the posterior column passing into the grey commissure ; p, fibres 

 of the sensitive roots going off to the lateral columns ; q, similar fibres entering the 

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

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

 radical fibres running horizontally forwards to the grey commissure ; «, large cells 

 of the anterior cornua (the puncta), inner group ; r, the same, outer group. 



