THE TISSUES 45 



current be suddenly allowed to pass into it or suddenly cut out 

 of it, or whether it is suddenly made stronger or weaker. 

 (Practical Physiology.) 



This method of stimulating muscle is constantly used in 

 medicine. It is a matter of no importance how the electricity 

 is procured, but most usually it is obtained either 



1st. Directly from a galvanic battery or electric main ; or 

 2nd. From an induction coil. 



If a galvanic battery or current from the main be used (1) 

 On making (closing) the current, and upon breaking (opening) 

 the current, a contraction results. While the current is flow- 

 ing through the muscle, the muscle usually remains at rest ; but 

 if the current is suddenly increased in strength or suddenly 

 diminished in strength, the muscle at once contracts. With 

 strong currents a sustained contraction galvanotonus may 

 persist while the current flows. (Practical Physiology.) 



It is the suddenness in the variation of the strength of the 

 current rather than its absolute strength which is the factor in 

 stimulating, as may be shown by inserting some form of 

 rheonome into the circuit by which the current may be either 

 slowly or rapidly varied. 



(2) If a current be made weaker and weaker, breaking ceases 

 to cause a contraction, while making still produces it. That is, 

 the stimulus on making is stronger than that on breaking. 



(3) The two poles do not produce the same effect. The 

 negative pole or kathode that coming from the zinc plate of 

 the battery causes contraction of the muscle on closing; while 

 the positive pole or anode causes contraction at opening. This 

 may be summarised as follows : 



1. Contraction on closing ; contraction on opening. 



2. Closing contraction stronger than opening contraction. 



3. Contraction at kathode on closing, at anode on opening. 



1. C.C C.O 



2. CC>CO 



3. CKC CAO 



How can these facts be explained ? 



