ELECTROTONIC CURRENTS 



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can be obtained in this way by dropping a nerve on an artificially 

 tetanized muscle. The beat of the heart causes usually only a 

 single secondary contraction when the sciatic nerve of a frog is 

 allowed to fall on it (p. 203). But when the diphasic variation is 

 well marked, as it is in an uninjured heart, there may be a secondary 

 contraction for each phase 

 i.e., two for each heart-beat. 

 Excitation of one muscle may in 

 the same way cause secondary 

 contraction of another with 

 which it is in close contact. 



Fig. 3o5. Secondary Contrac- 

 tion. The nerve of muscle M 

 touches muscle M' at a and b. 

 Stimulation of the nerve of M' 

 at S causes contraction of M. 



Fig. 306. Electrometer Record from Rab- 

 bit's Heart (Gotch). The heart was ex- 

 posed and beating in situ. Contacts, one 

 on base of right ventricle, the other on 

 right apex. The commencement of the 

 beat is on the left-hand edge of the dark 

 line V. The length of the dark line shows 

 the duration of the beat. Upward move- 

 ment signifies relative negativity (activity) 

 of the part at or near the base contact. 

 Time-trace at top, one-fifth second. 



The electromotive phenomena of the heart and of the central ner- 

 vous system are naturally included under those of muscle and nerve. 



Heart. Records of the electrical changes obtained with the 

 capillary electrometer or string galvanometer from the exposed 

 ventricles vary in certain details with the position of the two 

 contacts. When one contact is on the base of the ventricles (in the 

 rabbit) near the auriculo-ventricular groove, and the other on the 

 apex (Fig. 306), for each beat of the ventricle the electrometer record 

 shows (i) a sharp rise, indicating relative negativity (activity) of 

 the base ; (2) an equally sharp fall, indicating relative negativity at 

 the apex; (3) a slower but marked rise, indicating an increase or a 

 fresh development of relative negativity at the base; (4) a more 

 rapid fall, which returns 'first slowly, then quickly, until (i) follows 

 again (Gotch). The time between the beginning and the top of 

 rise (i) is bslieved to correspond to the time of transmission of the 



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