180 



ELECTRO-PHYSIOLOGY 



CHAP. 



changes of form (Fig. 76). .Fresh muscles, or those which, though 

 older, are still in a state of considerable tonic contraction, 

 generally appear quite insensible to induced currents. 



The advantages of a sustained passage of current over brief 

 " current impacts " is also seen in tetanising excitation periodically 

 repeated. Fick pointed out that the rapid make and break, 

 by hand, of an intrinsically effective constant current, generally 

 failed to excite smooth molluscan muscle yet the duration of 

 the single impacts here is considerable; if it is still further 

 lessened, stronger and stronger currents will be required to pro- 

 duce any excitation. This is especially striking in excitation 

 with a rapid succession of induced alternating currents, and Fick 

 states " that in the same circuit that closes the secondary coil of 

 an ordinary induction apparatus, a frog's muscle may fall into 

 the most lively tetanus, while the molluscan muscle shows 

 no sign of excitation," and that this even occurs with currents 



FIG. 76. Contraction curves of adductor muscle of Anodonta, excited by single make and break 

 induction currents (s and o) of increasing strength (a, at greatest distance of coil). 



that are strong enough to throw the muscles of the experimenter's 

 hand into tetanus (4). 



Engelmann's observations on the ureter (5) naturally fall 

 into line with these experiments on the smooth adductor muscle 

 of molluscs. Here, too, it is easy to demonstrate that the make 

 contraction occurs only when the duration of current exceeds a 

 certain limit, which is lower in proportion with the strength of the 

 current. This is plain from the accompanying table (Engelmann). 



Strength of current in rheochord 

 resistance. 



4000 

 500 

 50 

 25 

 15 

 12 

 11 

 10-5 



cm. 



Minimum closure, required to produce 

 contraction. 



quarter second. 



