THE NERVOUS SYSTEM. 367 



the gray central nucleus, and did not bend backwards until they reached 

 the posterior columns ; they were also well displayed in the lumbar en- 

 largement, between the substantia yelatinosa and the posterior commis- 

 sure. The horizontal radical fibres, also, which proceed to the lateral 

 columns, are often exceedingly numerous, although much less so, appa- 

 rently, than those which enter the posterior columns. The connection 

 of the gray commissures with a portion of the sensitive radical fibres, 

 is, as regards the posterior fibres, not difficult to be seen, these fibres, 

 in part, at least, running backwards along the posterior columns, and 

 being continued directly into the fasciculi of the substantia gelatinosa. 

 In the anterior gray commissure, I have also noticed, though not a direct 

 connection with the sensitive roots, still, fibres which, running horizon- 

 tally in a direction towards the summits of the posterior horns, entered 

 those processes. The commissural fibres are, besides, connected not 

 only with the sensitive roots, but also, and indeed quite evidently, with 

 the posterior columns, and less distinctly with the lateral ; from the 

 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 gray commis- 

 sures from the same columns on the other side. 



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

 the gray substance of the posterior horns. In the roots themselves 

 they still measure about 0*008 of a line ; in the substantia gelatinosa, 

 never more than 0-004, in the substantia grisea, 0-001-0-003 of a 

 line; in the gray commissures, not more than 0-0008-0-0012, in 

 the posterior and lateral columns, again, 0-0012-0-004 of a line; in 

 connection with which, however, it should be remarked, that, in this 

 situation, the increase of size in the fibres which enter these columns 

 horizontally, is not perceptible at their commencement, as is shown more 

 particularly in vertical sections, made in the direction 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 gray substance, a considerable number of nerve-fibres not 

 referable to the roots, and which until more is known about them may 

 be termed special spinal-medullary fibres. 



The filum terminale contains, so far as it is hollow, as a continuation 



