564 THE SPINAL COED. 



molecular disturbance which is the essence of the impulse ; that is to say, by 

 molecular resistance than by actual breaks of continuity in the nervous 

 matter. Indeed, we have some reasons for thinking that actual structural 

 continuity of nervous material 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 gray matter there are areas of white matter which do not 

 belong either to the nerve-roots as these are making their way into the gray 

 matter, or to any of the tracts which we have mentioned. These compris'e 

 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 de- 

 scending degeneration detailed above are taken out, seems at all events in the 

 higher parts of the cord (Fig. 127), relatively small, and future observations 

 may continue to still further 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 

 knowledge 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. But we may, at all events provisionally, assume that 

 these fibres serve in the main as commissures connecting the successive seg- 

 mental 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 gray matter of the intervening 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 com- 

 missure 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 gray matter of the isthmus forming the anterior and posterior gray com- 

 missures. 



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 

 gray 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 gray 

 matter. 



THE REFLEX ACTIONS OF THE SPINAL CORD. 



495. In the preceding portions of this work we have repeatedly seen 

 that though we can learn much concerning the working of an organ or tissue 

 or part of the body by studying its behavior when isolated from the rest of 

 the body, all the conclusions thus gained have to be checked by a study of 

 the behavior of the same organ or part while it is still an integral part of the 

 intact body. All the several organs and tissues are so bound together by 

 various ties that the actions of each depend on the actions of the rest ; and to 



