XXXII.] STIMULATION OF MUSCLE. 189 



the muscle to contract when the stimulus is applied to the nerve, 

 i.e., for indirect stimulation. 



(<;.) 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. The excitability of muscle to direct 

 stimulation is best done after the nerve -terminations have been 

 paralysed by curare (Lesson XXXIII.). 



(ii.) Constant Current. Connect the electrodes with two Daniell's 

 cells, placing a Hg-key in the circuit. Place the electrodes under 

 the nerve. Contraction occurs at make only, and at break only 

 if the preparation is very excitable, but there is no contraction 

 when the current is passing through the nerve. 



ADDITIONAL EXERCISES. 



3. Muscle Sound. 



(a.) Insert the tips of the index fingers into the auditory meatuses, forcibly 

 contracting the biceps muscles. A low rumbling sound is heard. 



(b.) When all is still at night, firmly close the jaws, and, especially if the 

 ears be stopped, the sound is heard. 



4. 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 telephone, and observe 

 that on doing so the muscle contracts vigorously. 



5. Rupturing Strain of Muscle and Tendon. 



(a.) Dissect out the femur and gastrocnemius with the tendo Achillis of a 

 frog. Fix the femur in a strong clamp on a stand, preferably one with a 

 heavy base. To the tendo Achillis tie a short stout thread, and hang a scale- 

 pan on to it. 



(b.) Place weights in the scale-pan, and note the weight required to rupture 

 the tendon or muscle. Usually the muscle is broken. The weight added will 

 be i kilo. , more or less, according to the size of the frog. 



(c.) Compare the rupturing strain of a frog's gastrocnemius which has been 

 dead for forty -eight hours. A much less weight is required. 



6. Dynamometers. 



(a.) Hand. Test the force exerted first by the right hand and then by the 

 left, by means of Salter's dynamometer. 



(b.) Arm. Using one of Salter's dynamometers, test the strength of the 

 arm when exerted in pulling, as an archer does when drawing a bow. 



