ON THE MAMMALIAN NERVOUS SYSTEM. 415 



indicates, however, a difference, since we have not as yet observed in the Cat or 

 Monkey any such very marked change in the excitatory effects in purely nerve 

 preparations as definitely dependent upon the degree of etherisation. 



The absolute determination of the true character of the minimal cord effect, and the 

 question to what extent it is to be considered as entirely due to the propagation of 

 nerve impulses along direct and continuous paths, unmixed with impulses generated 

 in nerve centres, might be settled by accurate experimental observations upon the time 

 relations of the cord electrical change, particularly the time of its development 

 relation to the seat of nerve excitation. Such an enquiry would doubtless involve the 

 use of Bernstein's differential rheotome, since the small size of the minimal effect is a 

 serious objection to the use of the much simpler method of recording by photography 

 the movement of the meniscus of a capillary electrometer. 



It may be mentioned incidentally at this point that the cord effect evoked by a 

 single electrical excitation of the sciatic nerve was, however, sufficiently pronounced to 

 cause a distinct movement in the mercury of our capillary electrometer. The move- 

 ment when magnified 300 times was just visible with a weak stimulus (coil, 500), and 

 was quite marked with a maximal stimulus (coil, 2000). It was always a single move- 

 ment, but appeared to the eye rather more prolonged in character than that which 

 was obtained in the case of the single nerve effect as displayed in the photographs of 

 our former paper.* 



It may be asked at this juncture upon what evidence we assume that the effect in 

 the cord, following the excitation of the sciatic nerve, is due solely to the arrival therein 

 of nerve impulses which have travelled up posterior or afferent fibres. The evidence, 

 which is supplied in full detail (in Chapter XL) upon the relations of the nerve fibres 

 and the nerve cells, shows that no electrical changes appreciable by the galvanometer 

 can be produced in the cord by even strong stimulation of the anterior or efferent 

 roots. 



It is thus clear that stimulation of the sciatic nerve generates nerve impulses which 

 enter the cord by the posterior roots only, although, owing to our ignorance of the 

 influence of the spinal ganglion, it cannot be assumed that the impulses are the same 

 in character as those generated by excitation of the posterior roots themselves. We 

 may now pass on to the consideration of the object of our experiments. 



The object of the investigation will be best set forth by considering the present 

 position of our knowledge of the relations of the spinal cord to the lumbar nerves, 

 and particularly to the afferent fibres of -those nerves. For if we are justified 

 in assuming that the electrical effect observed in the cord is due to the passage of nerve 

 impulses, and their arrival at the portion of the cord under observation, then, since 

 the method of observation is one which furnishes us with quantitative data, its 

 employment places within the experimenter's grasp a means of ascertaining to what 

 extent these impulses can be interrupted by definite section of any one tract of cord 



* ' Roy. Soo. Proc.,' loc. cit. 



