THE ELEMENTS OF NERVE PHYSIOLOGY 45 



persistence, after amputation, of sensations long associated 

 with the member that has now been lost. The experience 

 is said to be vivid and distressing. Spiritualistic theories 

 have been advanced with regard to it. But, after all, it 

 is just what we ought to expect. The stump contains 

 all the fibers that were formerly concerned in bringing 

 the impulses from the missing limb. If certain ones 

 are subjected to unusual tension or pressure in the read- 

 justment of the tissues attendant upon the healing process, 

 impulses may be started along the old channels. All the 

 associations of the resulting sensations will be with the 

 peripheral localities from which impulses used to arrive 

 over these routes. 



Summary. We are now in a position to sum up the 

 substance of the last few pages. The white matter of the 

 nervous system serves to conduct impulses which can 

 usually be considered quite lacking in individuality 

 to and from places in the gray matter. The paths in the 

 white matter are permanent. For the most part, at least, 

 they exist from infancy and are changed only by gross 

 pathologic processes. Conductivity, the course a fixed 

 one, is the distinctive property of the fibers as it is of 

 electric wires. If this is true, what shall we say of the 

 gray matter? We shall be inclined to credit it with the 

 power of reinforcement, as already pointed out, but 

 here too we must continue to emphasize the duty of con- 

 ducting. A most important difference between the con- 

 trasted kinds of nervous tissue is found in the varying 

 direction taken by impulses traversing the gray matter 

 at different times as compared with the inflexible be- 

 havior of the white. If we liken the nerve-fibers to wires, 

 as we have just done, we may find in a switch-board a 

 partial analogy to the gray matter. Wires and switch- 

 boards both serve to conduct currents, but the latter 

 permit the interruption and the resumption of the con- 

 duction and the most extensive shifting of connections 

 between channels of entrance and exit. Similarly, we 

 are brought to recognize that an inflowing stream of ini- 



