ELECTRICAL STIMULATION 



305 



As we increase the rate of change of current, by employing in the case 

 of induced currents more and more rapid alternations, we find that the 

 excitatory effect, 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 

 ' characteristic ' of an irritable tissue. 



FIG. 121. String galvanometer records of the change of current obtained by 

 opening the diaphragm in the rheonome (Fig. 120) at different rates. 

 (K. LUCAS.) 



A further investigation of the time relations of electrical stimuli 

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

 excitatory 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 increased 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.) 



CO 



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 this case the minimal stimulating current was 

 approximately doubled when the duration of the current was 



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