CHAP, ii.] THE BRAIN. 1095 



impulses along certain of the fibres of the posterior root make 

 their way upwards along the cerebellar tract, and there are some 

 reasons for regarding the vesicular cylinder and the cells which 

 represent this where it is not conspicuous in the regions of the cord, 

 as a relay between the two systems of fibres. There are also the 

 more scattered fibres of the ascending antero-lateral tract ( 567), 

 which probably is also an afferent tract, and therefore probably 

 also connected with the posterior roots ; but as we have seen our 

 knowledge of this tract is imperfect, though, if as some urge it 

 ends in the restiform body, we may perhaps consider it as similar 

 at least to the cerebellar tract, and treat the two as one. 



Thus there seem to be at least two main recognised paths, in 

 the form of tracts of fibres, for afferent impulses along the cord ; 

 one along the median posterior column, the other along the lateral 

 column in the cerebellar tract. The latter passes straight up to 

 the cerebellum by the restiform body, travelling along the same 

 side of the cord ; and any crossing of impulses passing along this 

 tract must take place before they enter the tract ; we have how- 

 ever no anatomical guidance for such a crossing. The other path, 

 along the median posterior tract, comes to end in the gracile 

 nucleus; it has indeed been urged that the gracile nucleus is 

 thus connected chiefly with the lower limbs and lower part of 

 the body, and that the analogous posterior root fibres from the 

 upper limbs and neck pass similarly into the cuneate nucleus, or 

 at least into the median division of that nucleus, but this cannot 

 be considered as proved. Moreover both the posterior columns, 

 median and external, bring to these nuclei fibres which have 

 started from some relay in the grey matter lower down, and 

 which are not fibres coming straight without any relay from 

 the posterior roots ; these however we cannot distinguish from 

 each other in their course beyond the nuclei. From the gracile 

 and cuneate nuclei the path onward is a double one, one broad, 

 one narrow. The broad path, the one having most fibres and 

 presumably carrying most impulses, leads to the cerebellum by 

 the restiform body; and here the path, previously continued 

 exclusively along the same side of the cord, becomes partly 

 crossed though remaining partly uncrossed, the sensory decussa- 

 tion in the bulb being the crossed and the other fibres passing 

 from the nuclei straight to the restiform body being the uncrossed 

 one ( 612) ; the uncrossed one we may perhaps look upon as 

 really an upper part of the cerebellar tract. The narrow path is 

 the fillet ( 634), by which some of the fibres from the nuclei are 

 continued on towards the cerebrum. This path is a crossed one, 

 the crossing taking place in the sensory decussation, and it carries 

 relatively few impulses, the chief increase in the size of the fillet 

 as it passes onward being due to fibres coming from structures 

 other than the gracile and cuneate nuclei. 



Hence of the sensory impulses travelling along continuous 



