90 PRACTICAL PHYSIOLOGY 



would not prevent the passage of a purely electric current. A 

 loss of excitability readily occurs if the nerve is allowed to dry, 

 but during this process there may be irregular fluctuations in the 

 excitability above and below the normal. 



Unipolar Excitation. Connect a battery to a coil so as to give 

 tetanising shocks ; connect a wire to one pole of the secondary 

 coil, and place its free end on the tongue. If the secondary coil 

 be moved completely over the primary, faint shocks will be felt. 

 The explanation of this phenomenon is that the making and break- 

 ing of the primary circuit causes free electricity to collect at the 

 end of the wire connected with the secondary coil ; when the 

 electromotive force of this charge is sufficient to overcome the 

 resistance of the tissues of the body, the circuit is completed through 

 the body, the floor and desk, and so back to the other pole of the 

 secondary coil. With the wire still on the tongue, touch the other 

 pole of the secondary coil with a moistened finger ; much more 

 powerful shocks are felt because a more direct circuit from one 

 pole to the other of the secondary coil has been provided. 



Repeat the experiment on a sciatic-gastrocnemius preparation 

 in the following way, with either tetanising or single-induction 

 shocks. Lay the preparation on a perfectly clean and dry glass- 

 plate and place a wire connected with one pole of the secondary 

 coil under the nerve ; no contraction of the muscle takes place 

 because the dry plate insulates the preparation and the secondary 

 circuit cannot be completed. Now touch the muscle with a wire, 

 the other end of which rests on a gas or water pipe ; the muscle 

 contracts because the circuit is completed through the earth. It 

 is not even necessary that the conductor should touch the prepara- 

 tion, for, if a moistened finger is brought as near the muscle as 

 possible without touching it, the muscle contracts, especially if 

 a moistened finger of the other hand touches the other pole of the 

 secondary coil. In this case the human body acts like a condenser 

 charged with electricity, which by its approach can stimulate 

 muscle or nerve. Further, if the nerve be ligatured between the 

 electrode and the muscle, or cut across and the two cut ends laid 

 over each other, which will prevent the passage of a nervous impulse 

 along it, contraction of the muscle is still produced, because the 

 discharge takes place along the whole length of nerve and muscle 

 between the electrode and the point by which the muscle is con- 

 nected to the earth, so that any irritable tissue in the course taken 

 by the charge is stimulated. 



If, however, the muscle and nerve preparation is laid on an ordi- 

 nary moistened muscle-board, the insulation is so slight that one 

 electrode, connecting the nerve and the secondary coil, will by 

 itself cause the muscle to contract. 



It is in order to guard against accidental stimulation of muscle 

 and nerve by unipolar action that a Du Bois key must always be 

 placed in the secondary circuit, and must always be kept closed 

 except when the tissue is being intentionally stimulated. The 



