18 ELEMENTARY PHYSIOLOGY. II. 



c. Direction of induction currents. Put the 

 secondary coil over the primary, and place the electrodes 

 on moistened neutral litmus paper. Send a series of break 

 currents through the electrodes, after a time the paper 

 will turn red at one pole and blue at the other (cp. p. 6). 

 Send in a series of make currents, the colouring will be 

 reversed. 



d. Unipolar stimulation. Disconnect one of the 

 electrodes from the short-circuit key and open the key. 

 Make and break the primary current, there will prob- 

 ably be no contraction. Push the secondary coil nearer 

 the primary ; at some position of the coil the make and 

 break of the primary current will cause contraction. 

 Repeat, touching the leg with wet finger; the contrac- 

 tion will be greater. Thus a break in the secondary cir- 

 cuit does not prevent the nerve being stimulated. It is 

 for this reason that an in-circuit key must not be used 

 in the secondary circuit. 



6. Rapidly repeated induction shocks. Arrange 

 the induction machine for tetanising currents (cp. p. 11). 

 Close the key of the secondary circuit. Make the primary 

 circuit. When the hammer is steadily oscillating open the 

 secondary key ; the leg is immediately thrust out straight 

 and kept rigid in tetanus. In a few seconds, break the 

 primary current, the limb at once becomes flaccid. 



7. Reaction of living and dead muscles. Have 

 ready a piece of faintly blue and a piece of neutral litmus 

 paper, and some hot water. 



a. Strip the skin from the frog's leg used for chemical 

 nerve stimulation ( 2). Cut off on one side the thigh 

 muscles, and press the cut surface on the litmus paper ; 

 the reaction will be faintly alkaline. 



