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A TEXTBOOK OF PHYSIOLOGY 



consists of a smooth amalgamated zinc rod dipping into a saturated 

 solution of zinc sulphate contained in a U-tube. Into the other limb 

 of the tube is inserted a glass flange carrying a plug of kaolin paste 

 made up with physiological saline solution. The kaolin plug is pulled 

 out to a point which serves as the electrode, or pieces of lamp-wick 

 soaked in the paste may be employed to make the contact. 



Electrical stimulation may be made either with a constant current 

 or with an induced current. With the former, the current from the 

 cell or battery is led by a make-and-break key direct to the muscle 

 preparation (Fig. 265). Stimulation is effected at the moment when 

 the current is caused to flow (at make) (Fig. 266), or stopped froni 



FIG. 265. PLAN OF THE USE OF A CONSTANT CURRENT TO STIMULATE. 



PIG. 266. CIRCUIT ARRANGED WITH SHORT-CIRCUITING KEY: KEY SHUT. OPENING 



KEY = MAKE SHOCK. 



FIG. 267. CIRCUIT ARRANGED WITH SHORT-CIRCUITING KEY : KEY OPEN. CLOSING 



KEY = BREAK SHOCK. 



flowing through the preparation (at break) (Fig. 267). There is no 

 ign of stimulation while the current is actually passing through 

 the muscle, provided the current be not too strong. If the current 

 be strong or the muscle injured, a long-continued contraction may 

 take place both in frog and human muscle. When muscles are 

 degenerating, it is found that the passage of even a relatively weak 

 current may cause this prolonged contraction, sometimes termed 

 " galvano-tonus." 



The contraction which occurs at the make is stronger than the 

 contraction at the break of the constant current. The make con- 

 traction starts from the kathode, the point where the electric current 

 leaves* the muscle; the break contraction starts from the anode, the 



