ii CHANGE OF FORM IX MUSCLE DURIXG ACTIVITY 131 



Yet we have here no true superposition of contractions, but the 

 first lift is of the same height as after a single effective stimulus. 

 At first, interruptions may still be seen in the tetanus curve, the 

 period of which is not, however, that of the stimulus, but longer, 

 intrinsic to itself, and determined by the specific nature of the 

 muscle -substance. Ranvier (18) obtained the same tetanus 

 curves from the ventricle of the frog's heart. It cannot be 

 doubted that this reaction of the cardiac bulbar muscles to 

 tetanising excitation stands in the closest relation with their 

 highly-developed faculty of rhythmical activity ; we know that 

 perfectly constant stimuli, e.g. chemical and mechanical, produce 

 rhythmical contractions of cardiac, and also under certain condi- 

 tions of striated skeletal, muscle during the entire duration of 

 their action ; this property is, however, much less developed in 

 the latter than in the former. 



We must assume that a series of single stimuli would 

 approximate the more closely in their physiological effect to the 

 action of a persistent stimulus, in proportion with the rapidity of 

 succession of the stimuli, so that it would not be surprising if, 

 under certain conditions, the effect of a succession of stimuli 

 corresponded with that of a persistent stimulus in striated skeletal, 

 as in cardiac, muscle. This does, indeed, appear to be the case, 

 and two phenomena especially are remarkable as claiming atten- 

 tion in this particular, i.e., on the one hand, the rhythmically 

 interrupted tetanus, on the other, the so-called initial contraction. 

 Kichet (4, p. 126) was the first to describe rhythmical alterations 

 in the curve of tetanised crab's claw-muscle, for which it appears 

 that weak and very frequent stimuli are essential. Schoenlein 

 (19) (Fig. 61, a, b) soon after made similar observations on 

 beetle muscles (dytiscus and hydrophilus). He obtained, on 

 exciting the muscles in the detached femur with induction 

 currents of low strength and high frequency, either rhythmical 

 contractions (dytiscus), or rhythmically interrupted tetani of 

 longer duration (hydrophilus, crab), or finally contractions, separ- 

 ated by pauses of rest at different intervals. The stimulation- 

 frequency in these experiments was usually 880 per sec., but the 

 phenomena may be observed at much higher frequencies. The 

 lower threshold is in the beetle 100 or 80, in crab as low as 30 

 per sec. 



As regards current strength, the rhythm varied between 



