COURSE OF THE FIBRES IN THE SPINAL CORD. 365 



fibres that traverse the middle line in the grey substance before 

 and behind the central canal. In my opinion, these fibres 

 entering into the posterior commissure are not to be regarded 

 as belonging directly to the posterior roots, but as fibres 

 passing backwards to ascend to the brain either in the vertical 

 fasciculi of the grey substance or in the posterior columns. If 

 this view be correct, and the analogous relations exhibited by 

 the anterior cornua are in favour of it, the following repre- 

 sentation or description of the course of the nerve fibres 

 entering the grey substance through the posterior roots may 

 be given : " A portion of the posterior root fibres break up 

 immediately after their entrance into that part of the grey 

 substance which possesses a nerve-fibre plexus into this 

 plexus itself; another portion runs more forward, and as it 

 proceeds the fibres undergo continuous subdivision and also 

 participate in the formation of the nerve-fibre plexus. This 

 plexus, the nodal points of which are formed by nerve cells 

 both of large and small size, is directly continuous with the 

 plexus of the anterior cornua. From it those nerve fibres take 

 their origin that cross the median line in the grey commissure 

 before and behind the central canal, then turn backwards, and 

 ascend towards the brain, partly in the vertical fasciculi of the 

 posterior cornua, and partly in the posterior columns, between 

 which two last sets of fibres are manifold, but hitherto inex- 

 plicable, relations." 



This description admits the complete decussation in the 

 spinal cord of the fibres reaching it by the posterior roots: 

 whether the decussation is really complete or only partial, a 

 portion of the fibres originating in the nerve plexus simply 

 running backwards towards the posterior columns, without 

 crossing the median line, cannot be determined on purely ana- 

 tomical grounds; both pathological observation, however, as 

 well as the experimental results obtained by the most com- 

 petent of all inquirers in this special research, M. Brown- 

 Se'quard, are decisively in favour of the occurrence of total 

 decussation. 



Finally, attention may here be especially called to two 

 points. 



1. The grey substance participates more freely in the con- 



