ON THE MAMMALIAN NERVOUS SYSTEM. 269 



i>u BOIS-REYMOND discovered to be invariable concomitants of excitatory processes. 

 As will subsequently be shown in the historical retrospect, it is well known, through 

 the researches of DU BOIS-REYMOND and others, that the fibres of the spinal cord, 

 just as nerve fibres in the peripheral trunks, are characterised by showing when 

 unexcited an electrical difference between their longitudinal surface and cross- 

 sections, and furthermore, that when excited a well-marked diminution of this 

 resting electrical state is produced in the fibres of the cord as in those of nerve 

 trunks. 



Now since such excitatory variations in the electrical state are presumably parallel 

 in time and amount with the presence in the nerve of the unknown processes termed 

 excitatory, which a series of stimuli evokes, it was reasonable to presume, if the 

 cortex were discharging a series of nerve-impulses at a certain rate down the 

 pyramidal tract, that there would be a series of parallel changes in the electrical 

 condition of the fibres in the cord tract, and that with a suitable apparatus for 

 responding to such changes these might be both ascertained and recorded. 



If this could be done, then the character of the discharge of the cortical centre 

 into the spinal cord would be, for the first time, definitely ascertained. 



As has been said before, the graphic method to some extent suggested the solution 

 of the problem, but the graphic method could not exclude, as this newer mode of 

 investigation does, the bulbo-spinal centres. Judging from the rate of contractions 

 of the muscles convulsed by excitation of the cortex, it was reasonable to expect that 

 the variations in the electrical condition of the pyramidal tract might intermit fifteen 

 or twenty times per second. If, therefore, they were to be observed or recorded, it 

 was obvious that some instrument would have to be used capable of quickly respond- 

 ing to very minute electrical differences succeeding one another at very short intervals 

 of time. The only instrument available for this task is LIPPMANN'S electrometer, 

 and, as we have stated in our previous communication, we had the advantage of the 

 assistance of Mr. G. F. BURCH in obtaining several very sensitive instruments. 



This instrument in addition presents the invaluable advantage of its movements 

 being easily recorded by photography, as originally described by BURDON SANDERSON 

 and PAGE. Such records are given in our previous paper in the ' Proceedings ' ; they 

 were from the first so definite, and so constant, that they enabled us at once to pass 

 on to a further development of the same method. It occurred to us that the method 

 afforded a means, not only of discovering the rhythm of the nerve disturbances as 

 they pass along the spinal channels, but also of investigating the line of communica- 

 tion existing between separated nerve centres, the mode of discharge of such centres, 

 and the determination of the direct paths, whether afferent or efferent in the 

 structure of the central nervous system. It is this last wider application of the 

 method to which we wish particularly to draw attention in the following pages, 

 inasmuch as although it has hitherto been possible by means of division or ablation 

 of certain portions of the central nervous system, e.g., the spinal cord, to trace by 



