PRACTICAL EXERCISES 655 



cross-wires in. Connect the commutator to one pair of the unpolar- 

 izable electrodes ('the polarizing electrodes'), as in Fig. 223. The 

 other pair of unpolarizable electrodes ('the stimulating electrodes') 

 are to be connected through a short-circuiting key with the secondary 

 of an induction machine arranged for tetanus. A single Daniell is 

 put in the primary coil. Pith a frog (brain and cord), make a muscle- 

 nerve preparation, pin the lower end of the femur to the cork plate 

 in the moist chamber, attach the thread on the tendo Achillis to the 

 lever connected with the chamber through the hole in the glass pro- 

 vided for this purpose, and arrange the nerve on the electrodes so 

 that the stimulating pair is between the muscle and the polarizing 

 pair. By moving the secondary, seek out such a strength of stimulus 

 as just suffices to cause a weak tetanus when the polarizing current 

 is not closed. Set the drum off (slow speed), and take a tracing of 

 the contraction. Then close the polarizing current with the Pohl's 

 commutator so arranged that the anode is next the stimulating 

 electrodes, i.e., the current ascending in the nerve. Again open the 

 short-circuiting key in the secondary; the contraction will now be 

 weaker than before, or no contraction at all may be obtained. Allow 

 the preparation two minutes to recover, then stimulate again, as a 

 control, without closing the polarizing current. If the contraction is 

 of the same height as at first, close the polarizing current with the 

 bridge of the commutator reversed, so that the kathode is now next 

 the stimulating electrodes. On stimulating, the contraction will now 

 be increased in height. (See Figs. 184, 185, p. 597.) 



(b) Arrange everything as in (a), except that one of the polarizing 

 electrodes is placed at each end, and the two stimulating elec- 

 trodes close together in the middle of the nerve. A large carbon 

 resistance (say 500,000 ohms) is introduced into the circuit of the 

 secondary coil, to prevent more than a very small fraction of the 

 polarizing current from passing through the coil. Seek out the 

 strength of stimulation which just causes contraction when the 

 polarizing current is not closed. Now close the polarizing current in 

 such a direction that the anode is between the stimulating electrodes 

 and the muscle. If no contraction occurs on stimulation, push up 

 the secondary towards the primary till the muscle contracts. Then 

 stop stimulation, open the polarizing current, and allow an interval 

 of two minutes. Now pass the polarizing current through the nerve 

 in the opposite direction, so that the kathode is between the stimu- 

 lating electrodes and the muscle. No contraction will be obtained 

 on exciting with the same strength of stimulus as caused contraction 

 when the anode was next the muscle. The kathode has diminished 

 the conductivity of the nerve ; and if four or five small Daniell cells 

 are put on in the polarizing circuit, no contraction may be obtained, 

 even with the coils close together, while the excitation will still pass 

 the anode and cause contraction. 



10. Pfliiger's Formula of Contraction (p. 597). To demonstrate 

 this, connect two unpolarizable electrodes, through a spring key and 

 a commutator, with a simple rheocord (Fig. 191), so as to lead off 

 a twig of a current from a Daniell cell The unpolarizable elec- 



