6i6 



A MANUAL OF PHYSIOLOGY 



2. Stimulation of Nerve and Muscle by the Voltaic Current (a) 

 Connect a Daniell cell through a key with a pair of electrodes on 

 which the nerve of a muscle-nerve preparation lies. Observe that 

 the muscle contracts when the current is closed or broken, but not 

 during its passage. 



Connect the cell with a simple rheocord, as shown in Fig. 191, so 



MB MB 



n 20 



FIG. 190. CONTRACTIONS CAUSED BY MAKE AND BREAK SHOCKS FROM AN 

 INDUCTION MACHINE. 



M, make, B, break, contractions. The numbers give the distance between the 

 primary and secondary coils in centimetres. 



that a twig of the current of any desired strength may be sent through 

 the nerve. As the strength of the current is decreased by moving 

 the slider S, it will be found that it first becomes impossible to obtain 

 a contraction at break. The current must be still further reduced 

 before the make contraction disappears, for the closing of a galvanic 

 stream is a stronger stimulus than the breaking of it. The break 

 or make contraction obtained by stimulating a nerve with an in- 



FIG. 191. SIMPLE RHEOCORD ARRANGED TO SEND A TWIG OF A CURRENT 

 THROUGH A MUSCLE OR NERVE. 



B, battery : R, rheocord wire (German silver) ; S, slider formed of a short piece of 

 thick indiarubber tubing filled with mercury ; K, spring key ; W, W, wires connected 

 with electrodes. 



duction-machine must not be confused with the break or make 

 contractions caused by the voltaic current. In the case of the 

 induction-machine, the break or make applies merely to what is done 

 in the primary circuit, not to what happens to the current actually 

 passing through the nerve. The current induced in the secondary at 

 make of the primary circuit is, of course, both made and broken in 



