292 PROTOPLASMIC ACTION AND NERVOUS ACTION 



When the shocks were lo per cent below the threshold, 

 the interval was much shorter. 



The summation-interval is thus longer the more 

 gradual the excitation-process (the longer the chronaxie) 

 of the tissue. It varies with temperature and with the 

 state of the tissue. Lucas finds the temperature- 

 coefhcient to be low (Qzo = ca. 1.3), a fact suggesting that 

 purely physical changes, e.g., diffusion-processes (which 

 have a similar temperature-coefficient), are chiefly con- 

 cerned in the return of the tissue to the normal after 

 a slight disturbance.^ The influence of the inorganic 

 salts is again highly interesting. The presence of Ca 

 shortens the summation-interval, just as it shortens the 

 minimal duration of the threshold constant current and 

 increases the rate of change required for stimulation by a 

 changing current.^ 



According to Lucas and Mines, the eft'ect of temper- 

 ature on the minimal duration of the threshold current 

 is the same as on the summation-interval. Such facts 

 again emphasize the distinction between the local change 

 produced by the stimulating agent and the propagated 



^ Cf. Lucas' discussion, op. cit., p. 473. It is noteworthy that the 

 time required for the return to the normal properties after complete 

 stimulation, as measured by the length of the refractory period, is much 

 longer than the summation-interval, and that the temperature-coefficient 

 of this return or recovery process is high (Qio = ca. 3) ; these facts indicate 

 that chemical processes play the chief part in the recovery from a com- 

 plete stimulation. The ''local change" may thus be of a purely physical 

 kind (e.g., polarization change), while in the complete or propagated 

 excitation the chemical factor is essential. This conclusion agrees with 

 the fact that the refractory period is much longer than the summation 

 interval. Apparently the former represents a period of metabolic and 

 structural restitution. 



* Lucas, op. cit., p. 472. 



