in ELECTRICAL EXCITATION OF MUSCLE 193 



Moreover, it is easy to show, as might be expected, that the 

 molluscan muscle in its relaxed state is highly sensitive to the 

 gradual entrance of current. If there is a rheochord in the 

 circuit, with as many cells as would be sufficient to set up a strong 

 closure contraction without the rheochord, an analogous change of 

 form in the muscle, corresponding with the commencement of the 

 persistent closure contraction, will invariably appear if the rheo- 

 chord slider is gently pushed forward from the zero as evenly as 

 possible. The contraction begins when the current has reached a 

 certain intensity, the curve rising the more steeply in proportion 

 with the rate at which the slider is pushed forward. We have 

 thus found it possible to record marked effects when the intensity 

 of current had been slowly increasing for two minutes ; the experi- 

 ment of course requires very sensitive preparations. 



We may conclude from the preceding data that every change 

 of form which can be termed a " twitch " in a suitable muscle, 

 requires for its effective stimulus a more or less rapid positive or 

 negative variation in current density, whether beginning at zero or 

 at a finite value; and since, as at once appears when a muscle 

 with parallel fibres is partially traversed by current (the case of 

 total excitation will be treated later), each twitch corresponds with 

 a wave of contraction spreading through the entire length of the 

 muscle, the transmission of excitation from the seat of direct 

 stimulation would seem in the last resort to be produced and con- 

 ditioned by a rapid variation in the current. Hence, while strength 

 of excitation depends fundamentally upon intensity, duration, and 

 density of current, the discharge of a wave of contraction depends also 

 upon the nature (steepness) of the increase of current intcmiti/ in the 

 muscle. 



These conclusions stand out more clearly from a simple, 

 graphic representation (Fig. 81), after Tick (4, p. 28 f.) The 

 abscissas indicate the times, the ordinates correspond with the 

 current intensity at the moment. While, in a given case, a passage 

 of current, such as is represented in Fig 81 (a), may fail to excite 

 both striated and smooth muscle, another process of current like 

 Fig 81 (&) may be an effective stimulus for the latter. In 

 order to produce the closure twitch in striated muscle a 

 steeper rise in the curve of current density is essential. In 

 variations of current, starting from and returning to zero, the 

 following cases are conceivable : a variation of the form (Fig. 81, c, c), 







