ELECTRICAL STIMULATION 271 



instead of increasing, begins to diminish and finally disappears, so that high- 

 frequency currents of enormous tension can, as in Tesla's experiments, be 

 led through the body without any apparent physiological effect. On 'the 

 other hand, by using more sluggish forms of irritable tissue, we may find that 

 even induction shocks are too rapid for effective excitation. Thus the red 

 muscles of the slow- moving tortoise react better to the slow make than to the 

 sudden break induction shock, and many forms of unstriated muscle are 

 unaffected by either make or break shock. There is in fact for each tissue 

 an optimum rate of change varying with the character of the tissue, at which 

 the current necessary to produce a response is at a minimum. This optimum 

 rate of change is spoken of by Waller as the c characteristic ' of an irritable 

 tissue. 



A further investigation of the time relations of electrical stimuli by 

 Keith Lucas has thrown important light on the character of the excitatory 



FIG. 120. String galvanometer records of the change of current obtained by 

 opening the diaphragm in the rheonome (Fig. 119) at different rates. (K. 

 LUCAS.) 



response itself. The difference between various excitable tissues is perhaps 

 best brought out by finding the minimum strength of current which will 

 excite at make and then determining how much this current must be in- 

 creased when it is broken at a very short interval of time after it has been 

 made. The following Table represents the relation between duration and 

 strength of current necessary to stimulate in the case of the sciatic nerve 

 of the toad : 



Duration of current (sec.) 



0070 

 0035 

 00087 

 00043 



Strength of current (volts) 

 086 

 091 

 119 

 179 

 245 



If we slightly alter the use by Waller of the word ' characteristic ' we 

 may take as the characteristic of the tissue the duration of the stimulus 

 at which the current necessary to stimulate was just double the minimum. 

 In the above case the minimal stimulating current was approximately 

 doubled when the duration of the current was limited to about -001 sec. 

 From a number of experiments of this description, Lucas gives the following 

 as the characteristic, or what he terms the " excitation times," of muscle 



and of nerve : 



