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. 51. 



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) finds that the tetanus curve of unfatigued muscle 

 (frog, toad) is " an equilateral hyperbola brought to an asymp- 



