XXXII.] STIMULATION OF MUSCLE. 187 



at which no response is obtained with unipolar stimulation, but a 

 response is obtained when the preparation is touched with finger. 

 Why is there a response ? Because by touching the preparation 

 one suddenly diminishes the resistance to the passage of the induc- 

 tion currents to earth. 



Or B. (a.) Set up a cell and induction coil with electrodes for 

 single shocks. Disconnect one of the electrodes of the secondary 

 coil, the other one being under the sciatic nerve or the nerve of a 

 nerve-muscle preparation which is insulated on a glass plate. If 

 the frog is on a frog-plate put the frog-plate on a dry beaker to 

 insulate it. No contraction occurs at make or break. 



(b.) Connect the disconnected electrode to a gas-pipe and so to 

 the earth. Contraction takes place at make or break. It is in order 

 to avoid unipolar stimulation that the Du Bois key is used to 

 short-circuit the secondary circuit. 



Or C. (a.) Connect the Daniell to the primary coil of the induction machine 

 either for single shocks or tetanus, introducing a Du Bois key in the circuit. 

 Connect one wire with the secondary coil, and attach it to one of the bind- 

 ing screws on the platform of the muscle-chamber, to which the nerve electrodes 

 are attached. See that the battery and induction machine are perfectly insul- 

 ated by supporting them on blocks of paraffin. 



(b.) Prepare a nerve-muscle preparation, and arrange it in the muscle- 

 chamber in the usual way, laying the nerve over the electrodes. One of the 

 electrodes will therefore be connected with the secondary circuit. 



(c. ) Make and break the primary circuit ; there is no contraction. 



(d.) Destroy the insulation of the preparation by touching the muscle, or 

 what does better, allow the brass support of the muscle to touch a piece of 

 moist blotting-paper on the inner surface of the glass shade of the chamber. 

 Every time the brass binding of the shade is touched, or the brass support 

 itself, the muscle contracts. Touch the secondary coil and contraction 

 results. 



LESSON XXXII. 



RHB ONOME TELEPHONE EXPERIMENT DIRECT 

 AND INDIRECT STIMULATION OF MUSCLE- 

 RUPTURING STRAIN OF TENDON MUSCLE 

 SOUND DYNAMOMETERS. 



1. Fleischl's Rheonome and Law of Excitation. This instru- 

 ment (fig. 112) is useful for showing Du Bois-Keymond's law, 

 that it is not the absolute intensity of a galvanic current flowing 

 through a nerve which excites it, but the rapidity of the variations 

 in the intensity of the current which excite a motor nerve. It 



