18 CHANGES OF EXCITABILITY IN NERVES 



To keep it moist, the nerve was placed with the tract to be 

 subjected to the mechanical excitation lying on a strip of filter- 

 paper soaked in a 0-6 % solution of common salt. When the 

 intrapolar tract was being- excited this strip included nearly the 

 whole distance between the electrodes. By this arrangement, 

 it is true, the branch of the current which passed through the 

 nerve was somewhat weakened, but this could be made up for by 

 means of a stronger current. Besides, in this way not only did 



Fig. 2. 



TIGEBSTEDT'S INSTRUMENT FOR MECHANICAL EXCITATION OP NERVES. 

 [The femur of the ' nerve muscle preparation ' is held by the clamp Cl. The nerve 

 lies on the anvil a. Whenever the current through the electro-magnet E is broken, 

 the edge of the hammer L falls on the nerve. The degree of excitation thus produced is 

 regulated by the position given to the sliding weight W. The distance of the seat 

 of excitation from the muscle can be varied by means of the spiral screw S. The whole 

 instrument is enclosed in a glass case so that the preparation is in moist air.] 



I succeed in keeping the nerve moist, but I obtained also a certain 

 measure of security against the sources of error which, in con- 

 sequence of the varying thickness of the nerve, attended Hermann's 

 experiments on the changes of excitability produced by a constant 

 current 1 . In some experiments specially indicated, where it was 

 important to have the polarising current as strong as possible, 

 I put no paper under the nerves. 



In those experiments in which paper was used the nerve was 

 never stimulated outside the sheet on which it lay. False results, 

 produced by the different strengths of the current in the rest of 



1 Hermann, Archiv fur die ges. Physiologie, vii, 1873, pp. 497-498. 



