370 ELECTRO-PHYSIOLOGY CHAP. 



also that the negative variation in the tract of muscle led off, 

 itself has a certain duration. For on pushing the slider along, the 

 deflections increase to a maximum, at which they persist for some 

 time ; but if it is pushed still further forward, no adjustment 

 will produce a deflection of the magnet. The slider may be 

 pushed over the whole graduated circle, without any repetition of 

 the galvanometer deflection, until it has passed back beyond its 

 first position, and reaches that in which the first negative deflec- 

 tion became apparent. The experiment therefore confirms the 

 conclusions derived from the observation of secondary tetanus, 

 to the effect that the negative variation of the muscle current 

 on tetanising corresponds not to a continuous diminution of 

 P.D. between longitudinal and cross -section, but to a discon- 

 tinuous waxing and waning in the rhythm of the excitation. 

 We see further that each single negative variation comes into 

 existence more rapidly than it vanishes ; graphically expressed, 

 its curve rises steeply to a maximum, and then sinks slowly 

 down again (cf. Fig. 114). If the rate of revolution of 

 the rheotome wheel, and the distance expressed in degrees 

 between the original position of the slider (when simultaneously 

 excited and led off), and that at which the first deflection 

 occurs, is known, it is easy to calculate the time (relatively 

 to the length of muscle between the point of stimulation and the 

 leading-off contact on the longitudinal surface) required by the 

 process of negative variation, in order to transmit itself from the 

 seat of excitation to the leading-off longitudinal contact. So too 

 the duration of the negative variation may be calculated from the 

 distance of the initial and final positions of the slider, and the 

 revolutions of the wheel. We should expect the duration of the 

 negative variation to increase with the distance between the 

 leading-off contacts, and this obviously corresponds with a certain 

 time -interval, which must be less in proportion as the tract 

 between the electrodes is shortened. But this anticipation is 

 not confirmed in experiment. The duration of the negative 

 variation is approximately equal, whatever the distance of the con- 

 tacts. This, however, means that the process which effects the 

 backward swing of the magnet is demonstrated by the galvano- 

 meter only while it passes over the base points of the leading-off 

 circuit in contact with the longitudinal surface, and not beyond 

 these points. Bernstein gives the velocity of the negative varia- 



