. Electro-physiology. Electrophysiology. 120 ELECTRO-PHYSIOLOGY CHAP. less modified degree, so that the phenomena described in muscle are only a special case of a universal principle. Viewed in the light of the relations which we have been urging between an increase of excitability produced by excitation, and the process of excitation itself, it is a matter of indifference whether the process be regarded as a true " summation " of ineffective into effective stimuli, or as increase of excitability produced by this summation. The following points with regard to form, process, and magni
. Electro-physiology. Electrophysiology. 120 ELECTRO-PHYSIOLOGY CHAP. less modified degree, so that the phenomena described in muscle are only a special case of a universal principle. Viewed in the light of the relations which we have been urging between an increase of excitability produced by excitation, and the process of excitation itself, it is a matter of indifference whether the process be regarded as a true " summation " of ineffective into effective stimuli, or as increase of excitability produced by this summation. The following points with regard to form, process, and magni- tude of tetanus contraction, and its dependence upon different variable factors, have been established by careful researches on the striated muscles of vertebrates and invertebrates. When, the stimuli are weak, and the frequency per sec. moderate (10— 12), the curve obtained from frog's muscles resembles Fig. FIG. 54.—Tetanus arising from, and resolving into, single twitches. The beginning and end of the tracing only are represented. In the omitted, central portion of 1'9 sec. the line traced by the muscle was horizontal. (Engelmann.) This will be recognised as very incomplete tetanus, with deep indentations, so that only in a minor degree can the muscle be said to be permanently shortened. The summits of the in- dentations lie almost horizontal. If the exciting induction currents are strengthened, or increased in frequency, the teeth become shorter and flatter, and the indentations less deep; the muscle reaches a much higher degree of permanent contraction (Fig. 52). Finally, the curve rises steeply from the beginning, and the indentation becomes negligible, disappearing altogether in complete tetanus (Figs. 53, 54). According to Kohnstamm (9) the tetanus becomes more incomplete with uniform frequency, in proportion with increasing strength of stimulus, since every incre- ment of stimulation accelerates the relaxation of the single con- traction (Fig. 54). Bohr (7)
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