METHODS OF STIMULATION 



25 



experiments, or varied in a definite and known manner. 

 Another great obstacle to be overcome in practice is the 

 avoidance of injury which is caused by the stimulus itself. 

 The application of stimulus above a critical intensity induces 

 a depression or abolition of excitability of the tissue. 



As the result of long investigation for the purpose of 

 securing various forms of quantitative stimulus, I find 

 that one mode of thermal and three modes of electrical 

 stimulation may be rendered practicable for our purpose. 

 These four different methods will be described in some 

 detail below. 



Electro-thermic Stimulation 



It is evident that touching the specimen with a hot 

 wire, though effective, is not a form of stimulus that is 

 capable of quantitative application or of repetition. It is 

 apt, moreover, unless 

 very great precautions 

 are taken, to injure the 

 tissue. 



The thermal mode of 

 stimulation can, how- 

 ever, be rendered prac- 

 ticable by the electrical 

 mode of the generation 

 of heat. A loop of fine 

 platinum-wire is made 

 to clasp round the peti- 

 ole which is to be ex- 

 cited, and is connected 



with an electrical circuit by means of fine flexible silver-wire 

 (fig. 6) . The circuit can be completed by a metronome inter- 

 rupter, the current from the battery flowing for a definite 

 length of time during, say, a single or definite number of beats 

 of the metronome. This produces a sudden thermal shock, 

 enough to cause excitation. Successive uniform stimuli can 



Fig. 6. — Electro- thermic stimulator for 

 uniform stimulation ; metronome em- 

 ployed in place of key k, for closing 

 circuit for definite length of time. 



