64 



PKACTICAL PHYSIOLOGY 



response to the single stimulus is not so great as that due to the two 



stimuli. In other words, there has 

 been a summation of stimuli during 

 the refractory period. In the same 

 way subminimal stimuli can be 

 summated, but two maximal stimuli 

 are summated only when they 

 follow each other after an interval 

 of less than T J^th second. 



As has been pointed out on p. 25, 

 when a ' striker ' passes over the 

 naked wire, there is both a make 

 and break of the primary circuit; 

 consequently in these experiments 

 the muscle really receives four 

 induction-shocks, of which, accord- 

 ing to the position of the secondary 

 coil, all four might be individually 

 subminimal, or the two break-shocks 

 might be alone effective, or all four 

 might be effective. In order to 

 deal with the summation of two 

 break-shocks alone, it is usual to 

 perform these experiments with 

 the following special piece of 

 apparatus. 



The Spring or Trigger Myograph. 

 (Fig. 63). It consists of a heavy 

 metal base which is clamped to the 

 bench. The essential part of the 

 apparatus is an oblong metal frame 

 carrying a smoked glass plate, the 

 recording surface, which is shot 

 on two horizontal wires past the 

 writing points. In order to prepare 

 the apparatus for use, the frame is 

 pulled to one side by one of the 

 arms attached to it ; this compresses 

 a spring on the other arm, and the 

 frame is held in position by a catch 

 or trigger. When the catch is re- 

 leased the spring gives the frame and 



