ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 123 



has little effect upon the position of the summit of the twitch, 

 and in correspondence with this the difference between height 

 of tetanus and height of twitch vanishes (Fig. 55, &). This 

 is intelligible when it is remembered that at low tension the ex- 

 ternal conditions of the process of contraction cannot be intrinsic- 

 ally altered by the support. Tetani lower than the single twitch 

 are frequent in fatigued muscle (Fig. 57). When a muscle has 

 had a short rest after a prolonged series of contractions, the first 

 twitch on the renewal of excitation is abnormally high in 

 proportion with the following (Buckmaster's " initial " twitch), 

 and this reversal of relation continues even if the muscle is 

 supported. 



At present there are insuperable difficulties in the way of any 

 adequate explanation of all these relations, and, in particular, of 



FIG. 57. Tetanus and single twitch of a fatigued and curarised muscle ; load, 10 grs.; rate of 

 stimuli, O'l sec. (v. Frey.) 



exact analysis of the process of tetanus, as is readily understood 

 when we consider the number of different factors in the tetanus 

 curve. The staircase appearance, superposition in Helmholtz's sense, 

 the effect of internal supporting, as well as fatigue and contracture, 

 all take more or less share in the process of tetanus (v. Frey). 



Another and probably important factor is, that in the majority of 

 cases a muscle is not a physiological unit, but represents a mixture 

 of at least two functionally different elements, which can hardly 

 be supposed to act simultaneously and uniformly. This leads 

 to the further question of the dependence of tetanic excitation 

 upon the nature of the muscle. Here, in the first place, we must 

 consider the widely varying duration of contraction in different 

 muscles, or the different fibres of the same muscle. An uninter- 

 rupted tetanus, where the twitches are superposed as above, can 

 of course be expected only when the interval of stimulation is 



