PRACTICAL EXERCISES 



783 



contraction, let the drum move a little before opening the key again, 

 so that the lines corresponding to make and break may be separated 

 from each other. If there is still no contraction at make, go on moving 

 the secondary up, a centimetre (or less) at a time, till a make con- 

 traction appears. When the coils are still further approximated, the 

 make may become equal in height to the break contraction, both being 

 maximal i.e., as great as the muscle can give with any single shock 

 (Fig. 276). 



(c) Attach a thin insulated copper wire to each terminal of the 

 secondary. Loop the bared end of one of the wires through the tendo 



Fig. 276. 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. 



Achillis, and coil the other round the pin in the femur, so that the shocks 

 will pass through the whole length of the muscle. Repeat the experi- 

 ment of (b), with direct stimulation of the muscle. 



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

 nect 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 con- 

 tracts when the current is closed or broken, but not during its passage. 



Fig. 277. 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. 



Connect the cell with a simple rheocord, as shown in Fig. 277, so 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 

 tho make contraction disappears, for the closing of a galvanic stream 

 is a stronger stimulus than the breaking of it. The break or make con- 



