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PHYSIOLOGY. 



ELECTRICAL EXCITANTS. This form of stimulus is surely the 

 most important to study and is, perhaps, the one that is most com- 

 plex. The electrical stimulus may consist of either the constant or 

 interrupted current. The stimulation of the nerve may be direct, 

 as when the electrodes are applied to the nerve. There are two 

 kinds of currents used: the induction current and the galvanic cur- 

 rent. I shall take up the constant current. 



ELECTROTONUS. When a constant current traverses a nerve it 

 alters its excitability, conductivity, and electromotivity. This is 

 called electrotonus. The part of the nerve affected by the positive 

 pole is said to be in a state of anelectrotonus, the part altered by 

 the negative pole to be in a state of katelectrotonus. Intrapolar 



A 

 fr 



N f G 



Fig. 268. Scheme of Electrotonic Excitability. 



The nerve (jV-n) is traversed by a constant current in the direction of the 

 arrow. The curve shows the degree of increased excitability in the neighbor- 

 hood of the cathcde (B) as an elevation above the nerve; diminution at the 

 anode (A) as a depression. The curve i-h-g shows the degree of excitability 

 with a strong current; the curve f-e-d with a medium current, and the curve 

 c-b-a with a weak current. A, is anode. B, is cathode. 



means between the electrodes or poles; extrapolar is outside the 

 poles. Descending current is down the nerve to the muscle; ascend- 

 ing current is from the muscle up the nerve. By the action of the 

 constant current on the nerve-muscle preparation at the time of 

 making and breaking of the same we have the contraction law of 

 Pfliiger. As an aid to memory we shall call the contraction "yes;" 

 no contraction, "no." 



Descending current. Ascending current. 



Strength of current. Make. Break. Make. Break. 



Weak Yes No Yes No 



Moderate Yes Yes Yes Yes 



Strom Yes No No Yes 



EXPLANATION OF PFLUGER'S CONTRACTION LAWS. These laws 

 are explained by the fact that a sudden increase of excitability at 

 the kathode at the make of a current, or a sudden change of excita- 



