156 EXPERIMENTAL PHYSIOLOGY. 



(a.) Connect two Darnell's cells with the binding screws, 

 A and B, introducing a du Bois key in one wire. Attach 

 the electrodes, introducing a du Bois key to short-circuit 

 them, to the binding screws, and D. Fill the groove with 

 a saturated solution of zinc sulphate. 



(b.) Arrange the nerve of a nerve-muscle preparation in 

 the usual way over the electrodes (Lesson XX VIII. , 4). 

 Pass a constant current through the nerve, observing the 

 usual effects, viz., contraction at make or break, or both, but 

 none when the current is passing. Then suddenly rotate 

 the handle with its two zinc arms ; this is equivalent to a 

 sudden variation of the intensity of the current; the current, 

 of course, continuing to pass all the time. The muscle sud- 

 denly contracts. 



2. Telephone Experiment. 



(a.) Arrange a nerve-muscle preparation with its nerve 

 over a pair of electrodes. Connect the latter with a short- 

 circuiting du Bois key. To the key attach the two wires 

 from a telephone. 



(b.) Open the short-circuiting key ; shout into the tele- 

 phone, and observe that on doing so the muscle contracts 

 vigorously. 



3. Direct and Indirect Stimulation of Muscle. When the 

 stimulus is applied directly to the muscle itself, we have direct 

 stimulation ; but when it is applied to the nerve, and the muscle 

 contracts, this is indirect stimulation of the muscle. 



(a.) Arrange a nerve-muscle preparation, and an induc- 

 tion machine for single or interrupted shocks (Lesson 

 XXVIII., 1). 



(b.) Test first the strength of current as measured by 

 the distance between the secondary and primary coils 

 which causes the muscle to contract when the stimulus is 

 applied to the nerve i.e., for indirect-stimulation. 



(c.) Then with the secondary still at the same distance 

 from the primary, try if a contraction is obtained on 

 stimulating the muscle directly. It will not contract, but 

 make the current stronger, and it will do so. 



