CHAP, i.] THE SPINAL CORD. 899 



the whole groundwork. The nuclei of the cells of the anterior 

 horn with the cell-substance adjacent to each and the cells of the 

 ganglia on the posterior root probably govern the nutrition, and 

 so the functional activity of the groundwork as well as of the 

 issuing and entering fibres ; but there appears to be as yet no 

 convincing evidence of any other peculiar powers confined to the 

 cells and absent from other parts of the groundwork. We may 

 add that, in accordance with this view, the other cells of the grey 

 matter, such as those of the vesicular cylinder, are to be regarded 

 as of importance for governing the nutrition of fibres, commissural 

 and others, starting from the spinal segment, and of the part of 

 the groundwork from which by their mediation the fibres start, 

 rather than for determining the functions of the groundwork of 

 the segment or of the fibres receiving impulses from it. 



580. The segmental groundwork of grey matter belonging 

 to each pair of spinal nerves is so fused with that of all the 

 other pairs, as to form along the whole length of the cord a mass 

 of grey matter which appears, under certain circumstances at all 

 events, to be continuous in the sense that impulses may pass in 

 all directions along it. But each spinal segment is in addition 

 connected by means of tracts of white matter with parts more or 

 less distant. The crossed pyramidal tract is such a longitudinal 

 commissural tract, connecting apparently each spinal segment in 

 succession with a certain part of the cortex of the cerebrum. We 

 have reason to think, as we shall see later on, that impulses 

 descending this or that fibre or group of fibres of this tract give 

 rise to the issue of motor impulses along this or that fibre or 

 group of fibres of an anterior root. We do not at present know 

 what is the exact manner by which the fibre in the pyramidal 

 tract is connected with the fibre of the anterior root. It seems 

 certain, however, that the connection is not in the form of a fibre 

 isolated from the rest of the grey matter, continuing, so to speak, 

 the pyramidal fibre into a cell of the anterior horn whence the 

 fibre of the anterior root issues. Most probably the pyramidal 

 fibre makes connections with the segmental groundwork spoken 

 of above, whether with or without the intervention of a cell we 

 cannot at present tell. The direct pyramidal tract is a like 

 tract of less extent downwards, and the less known antero-lateral 

 descending tract is probably of a similar nature. 



The cerebellar and antero-lateral ascending tracts are in like 

 manner to be regarded as longitudinal commissures between the 

 successive spinal segments below and some part of the brain above. 

 We have reason to think that these tracts convey upward impulses 

 of a nature which may be called afferent, and are therefore in some 

 way probably connected with the posterior roots. We do not 

 know as yet the exact nature of the connection ; but probably in 

 those cases also, the commissural fibres are united not directly to 

 the posterior fibres, but indirectly by means of the segmental 



