416 SPECIAL HISTOLOGY. 



anterior portions of which, adjoining the base of the posterior 

 horns, arched fasciculi pass into the commissures and become 

 intermixed with the other commissural fibres (fig. 141, o and I). 

 These fibres probably pass over to the opposite side, into the 

 commissures connected with the posterior roots ; in which 

 case, like the anterior halves of the cord, a decussation of fibres 

 also takes place in the posterior commissure. In accordance 

 with what has been remarked, the sensitive roots derive their 

 fibres principally from the posterior and lateral columns (the 

 posterior halves) on their own side, and probably also through 

 the grey commissures from the same columns on the other 

 side. 



The fibres of the sensitive roots also decrease in size as they 

 traverse the grey substance of the posterior horns. In the 

 roots themselves they still measure about O008'" ; in the 

 substantia gelatinosa, never more than 0'004'" ; in the sub- 

 stantia grisea, O'OOl — 0003'"; in the grey commissures, not 

 more than 0*0008 — 0*001 2'" ; in the posterior and lateral 

 columns, again, 00012 — 0-004'"; in connection with which, 

 however, it should be remarked, that, in this situation, the 

 increase of size in the fibres which enter these columns hori- 

 zontally, is not perceptible at their commencement, as is 

 shown more particularly in vertical sections, made in the direc- 

 tion from within to without, through the posterior cornua. The 

 variation in diameter may even be directly observed, in many 

 fibres in this situation; as, for instance, at the entrance of the 

 roots into the gelatinous substance. 



Besides these fibres, belonging to the motory and sensitive 

 roots, there exist, in the grey substance, a considerable number 

 of nerve-fibres not referable to the roots, and which until 

 more is known about them may be termed special spinat- 

 meduUary fibres. 



The filum terminate contains, so far as it is hollow, as a 

 continuation of the grey substance of the cord, a grey, soft 

 substance, consisting chiefly of round, nucleated, pale cells, 

 like nerve-cells, and measuring 0005 — O'OOG"'. Besides 

 these there occur, in its upper part among the cells, true dark- 

 bordered nerve-fibres, of various, and for the most part small 

 diameter, and also numerous, fine, pale fibres, the nature of 

 which is not clear to me ; that is to say, whether they are 



