128 IRRITABILITY 



A sharp contrast to this type is formed by the other extreme 

 as represented by that of the meduUated nerve. As an indicator 

 of the course of excitation we will take the action current in 

 an isolated nerve of the frog. If this is stimulated at one end, 

 we can test the intensity of the conducted excitation by leading 

 off the action current from two points at varying distances from 

 the one influenced by the stimulus. Since the classical discovery 

 of Du Bois-Rcymond of the action current of the nerve, we know 

 that in the fresh medullated nerve, if observed under favorable 

 experimental conditions, no decrement of intensity of excitation 

 during its course from the point of stimulation along the length 

 of the nerve can be demonstrated.^ If unpolarizable electrodes 

 are applied to a nerve in such a position that they are equidistant 

 from the cross section and are connected with apparatus for 

 testing the current, it will be found that there exists an "unwirk- 

 same Ableitung" in the sense of Du Bois-Reymond, that is, in 

 which there is no demarcation current. When a tetanizing cur- 

 rent is applied to one end of the nerve, no difference of potential 

 between the two nonpolarizable electrodes is observed, which 

 indeed would be the case if excitation with its current of 

 action would have a decrement on its way from one to the other 

 point of leading off the current. This fact, which has been repeat- 

 edly confirmed, shows us that the medullated nerve, under normal 

 conditions, conducts excitation without a perceptible decrement 

 of the intensity. 



This specific property of a medullated nerve is in conformity 

 with the conditions in connection with the rapidity of conduc- 

 tivity. Since Helmholtc' has devised the method for measuring 

 the rapidity of conduction in the nerve, this investigator himself 

 and numerous others have studied the rate in different nerves.^ 

 Helmholtz found the rate for motor nerves of the frog to be 

 27 meters per second, for the sensory nerves of man 60 meters, 



1 Du Bois-Reymond: "Untersuchungen fiber tierische Electricitat." II Band. 1849. 



2 H. Helmholtz: "Messungen fiber den zeitlichen Verlauf der Zuckung animalischer 

 Muskeln und die Fortpflanzungsgeschwindigkeit der Reizung des Nerven." Muller's 

 Archiv. 1850. 



The same: "Messungen fiber die Fortpflanzungsgeschwindigkeit der Reizung in den 

 Nerven." Zweite Reihe, Mfiller's Arch. 1852. 



3 Compare: Hermann: "Handbuch der Physiologic." II, 1 Leipzig 1879. 



