A sharp contrast to this type is formed by the other extreme as represented by that of the medullated 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-Reymond_ 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.[82] 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 “unwirksame Ableitung” in the sense of _Du Bois-Reymond_, that is, in which there is no demarcation current. When a tetanizing current 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 repeatedly confirmed, shows us that the medullated nerve, under normal conditions, conducts excitation without a perceptible decrement of the intensity._ [82] _Du Bois-Reymond_: “Untersuchungen über tierische Electricität.” II Band. 1849. This specific property of a medullated nerve is in conformity with the conditions in connection with the rapidity of conductivity. Since _Helmholtz_[83] 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.[84] _Helmholtz_ found the rate for motor nerves of the frog to be 27 meters per second, for the sensory nerves of man 60 meters, and the motor nerves of man 34 meters. Other investigators have obtained quite different results; _Hirsch_, for the sensory nerves of man, 34 meters; _Schelske_, for the same, 25–33 meters; _De Jaager_, 26 meters; _v. Wittich_, 34–44 meters, and _Kohlrausch_, 56–225 meters; _v. Wittich_ for the motor nerves of man, 30 meters; _Piper_[85] finally in the most recent investigations about 120 meters in the second. [83] _H. Helmholtz_: “Messungen über den zeitlichen Verlauf der Zuckung animalischer Muskeln und die Fortpflanzungsgeschwindigkeit der Reizung des Nerven.” Müller’s Archiv. 1850. The same: “Messungen über die Fortpflanzungsgeschwindigkeit der Reizung in den Nerven.” Zweite Reihe, Müller’s Arch. 1852. [84] Compare: _Hermann_: “Handbuch der Physiologie.” II, 1 Leipzig 1879.