118 ELECTRO-PHYSIOLOGY CHAP. 



it becomes intelligible that under certain conditions the height of 

 a summated twitch may far surpass that of its two components. 



It was shown above that the magnitude of interval between 

 each pair of stimuli must not exceed a certain limit, if the bene- 

 ficial effect of the preceding stimulus is to be observed upon its 

 successor, and it is intelligible that under some conditions tetanus 

 may be set up in the muscle, in consequence of a rapid succession 

 of weak stimuli, although these in themselves would produce no 

 visible change of form in the muscle. The intensity and 

 frequency of stimulation necessary to produce such a summation 

 (Richet's addition latente) must of course depend upon the nature 

 of the muscle. As a general rule, sluggishly reacting muscle is 

 more predisposed to summation of stimuli than quick muscle, 



FIG. 50. A, Simple twitcli (muscle of Crab); A'l, summated twitch, from two closely 

 approximated stimuli of the same magnitude as A. (Richet.) 



which tallies with the rapid expiration of all phenomena of 

 excitation in the latter, since the persistence of any kind of 

 change in the muscle-substance resulting from a stimulus is the 

 necessary condition of a subsequent heightening of excitability. 

 The comparatively sluggish striated muscle of the heart may be 

 indicated as peculiarly suited to summation effects in the above 

 sense. Basch (5) showed that subliminal, single, electrical 

 stimuli, inadequate in themselves to produce any contraction, will 

 gradually (addition latente) increase the excitability of the heart - 

 muscle (frog) if led into it at short intervals, until finally con- 

 tractions will be discharged. Engelmann (6) made similar obser- 

 vations on the bulbus aortse of the frog's heart, which also exhibits 

 unmistakable effects of summation when rhythmically excited ; 

 the most obvious instances, however, are in smooth muscle. Here 



