PRACTICAL EXERCISES 817 



with the side-cups of a Pohl's commutator with cross-wires in. Con- 

 nect the commutator to one pair of the unpolarizable electrodes (' the 

 polarizing electrodes '), as in Fig. 314. 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 provided 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 a 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, 



Fig. 314. Arrangement for showing Changes of Excitability produced by the Voltaic 

 Current. M, muscle; N, nerve; E lt E 2 , electrodes connected with secondary 

 coilS; E 3 , E 4 , unpolarizable electrodes connected with Pohl's commutator (with 

 cross-wires) C; B', 'polarizing' battery; B, 'stimulating' battery in primary 

 circuit P; K, K", simple keys; K', short-circuiting key. 



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 stimu- 

 lating, the contraction will now be increased in height. (See Figs. 266, 

 267, p. 760.) 



9. Pfliiger's Formula of Contraction (p. 762). To demonstrate this, 

 connect two unpolarizable electrodes, through a spring key and a 

 commutator, with a simple rheocord (Fig. 277, p. 783), so as to lead 

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

 trodes are placed in a moist chamber. A muscle-nerve preparation 

 is arranged with the nerve on the electrodes and the muscle attached 

 to a lever. The effects of make and break of a weak current, ascending 

 and descending, can be worked out with the simple rheocord. The 

 effects of a medium current will probably be obtained with a single 

 Daniell connected directly with the electrodes through a key. The 

 effects of a strong current will be got when three or four Daniells are 

 connected with the electrodes. Care must be taken to keep the prepara- 

 tion in a moist atmosphere, and more than one preparation may be 

 needed to verify the whole formula. 



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