CHAP. L] THE SPINAL CORD. 901 



is not essential to functional continuity, that a nerve fibril for 

 instance may produce its due effect on another nerve fibril or 

 on a nerve-cell, if sufficiently in contact with it, though the 

 microscope fails to demonstrate actual continuity. 



But besides the grey matter there are areas of white matter 

 which do not belong either to the nerve roots as these are making 

 their way into the grey matter, or to any of the tracts which we 

 have mentioned. These comprise the strands of fibres which do 

 not undergo either ascending or descending degeneration when 

 parts of the spinal cord are injured or diseased. The area of 

 white matter left when all the various tracts of ascending and 

 descending degeneration detailed above are taken out, seems, at 

 all events in the higher parts of the cord (Fig. 104), relatively 

 small, and future observations may continue still further to reduce 

 it ; but it must be remembered, that none of the above-mentioned 

 tracts are 'pure'; they are all more or less mixed up, and some 

 largely mixed up, with fibres which do not degenerate. Our know- 

 ledge is at present too scanty to allow us to make any statement 

 with confidence concerning the function either of the fibres 

 forming the white matter not yet marked out into tracts, or of the 

 fibres scattered among the acknowledged tracts. Bat we may, at 

 all events provisionally, assume that these fibres serve in the main 

 as commissures connecting the successive segmental mechanisms 

 with each other; we may conclude that changes taking place in 

 one segmental mechanism can by means of these fibres produce 

 correlated changes in some other distant segmental mechanism, 

 without calling into action any of the grey matter of the inter- 

 vening segmental mechanisms. 



The commissures which we may suppose to be thus furnished 

 by white matter are longitudinal commissures connecting the 

 segmental mechanisms of the same lateral half of the spinal 

 cord with each other. A transverse connection between the two 

 lateral halves is afforded in some measure by the anterior white 

 commissure. We shall see, however, later on reasons for thinking 

 that many impulses besides those passing along the anterior 

 commissure cross from one side of the cord to the other ; and 

 these whether they pass along distinct fibres or along the general 

 groundwork must travel by the grey matter of the isthmus form- 

 ing the anterior and posterior grey commissures. 



Thus, as far as we can see at present, the spinal cord consists of 

 a series of segmental mechanisms with their respective afferent 

 and efferent roots (the grey matter of the several segments being 

 continuous along the cord), of encephalic ties of white matter 

 between the several segments and the brain, of longitudinal 

 commissural tracts connecting together the several segmental 

 mechanisms, and of transverse commissures running largely in 

 the grey matter. 



