62 VETERINARY PHYSIOLOGY 



matter of no importance bow the electricity is procured, but 

 most usually it is obtained eitber — 



1st. Directly from a galvanic battery, accumulator, or 



electric main ; or 

 2nd. From an induction coil. 



Int. Galvanic Stimulation. 



A. Exposed Nerve. 



Tbe sciatic nerve of the frog passing to the gastrocnemius 

 muscle may be placed upon tbe wires from a galvanic 

 battery, and the contraction of the muscle may be taken 

 as the index of tbe stimulation of the nerve. 



It will be found that — {a) On making the current, 

 and upon breaking tbe current, a contraction results. 

 While the current is flowing through the nerve, 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 Plcysiology). 



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. This fact is of great 

 importance in applying galvanic currents in the treatment 

 of various diseases in the human subject. Great care is 

 necessary to increase sloivly and to decrease slowly the 

 strength of the current, or painful stimulation may be pro- 

 duced {Practical Physiology). 



(b) The stimulus on making is stronger than that on 

 breaking, so that, if a current be made weaker and weaker, 

 breaking ceases to cause a contraction, while making still 

 produces it {Practical Physiology). 



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

 negative pole or cathode stimulates on making ; while the 

 positive pole or anode stimulates at breaking. This may 



