742 A MANUAL OF PHYSIOLOGY . 



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, 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 253, 254, p. 683.) 



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

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

 close together in the middle of the nerve. A large carbon re- 

 sistance (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 elec- 

 trodes and the muscle. If no contraction occurs on stimulation, 

 push up the secondary towards the primary till the muscle contracts. 

 Then stop the 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 stimulating electrodes and the muscles. 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. 



9. Pfliiger's Formula of Contraction (p. 684). To demonstrate 

 this, connect two unpolarizable electrodes, through a spring key 

 and a commutator, with a simple rheocord (Fig. 260, p. 705), .so as to 

 lead off a twig of a current from a Daniell cell. The unpolarizable 

 electrodes are placed in a moist chamber. A muscle-nerve prepara- 

 tion 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 elec- 

 trodes 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 preparation in a moist atmosphere, 

 and more than one preparation may be needed to verify the whole 

 formula. 



