LABORATORY WORK 205 



The preparation may be most conveniently mounted on the 

 cork plate of a " myograph," which is a flat board with a bell- 

 crank lever attached. Put a pin through the end of the femur 

 into the cork. Tie the thread on the tendon to the upright of 

 the lever. Lay the nerve on electrodes, which may be held in 

 place by means of a lump of plasticine. A weight of 10 or 20 gms. 

 is hung on the lever near its axis. 



For stimulating electrically, some form of induction coil is used. 

 The secondary coil is wound on a bobbin separate from the primary 

 coil, so that it can be placed at different distances for adjusting the 

 strength of the stimulation. There is an automatic interrupter to 

 give shocks in continuous series. Such coils are sold by dealers in 

 physiological or electro-therapeutical apparatus. 



Electrical stimuli of moderate intensity do no damage to the 

 nerve, whereas it is difficult to avoid killing it by other forms. 

 Hence these latter can only be applied once at a particular spot. 

 The end of the nerve furthest away from the muscle must be used 

 first. A gentle tap with the back of a scalpel serves as a mechanical 

 stimulus, a crystal of salt as a chemical one, a heated wire as a 

 thermal one. 



Notice that the excised muscle does work in raising a weight. 

 It may be said that the weight falls again, so that no actual external 

 work is done. But when it is raised, a support may be slipped 

 under it, and, after the muscle has relaxed on cessation of stimula- 

 tion, the thread to which the weight is attached, which will now be 

 lax, may be shortened, and then the next stimulation raises the 

 weight still further. With patience the process may be repeated 

 many times until the muscle becomes fatigued. An automatic 

 " work-collector " on the principle of the ratchet wheel is constructed 

 to perform the operations described above. 



Formation of Acid 



Cut across a muscle which has not been stimulated, and press 

 a piece of neutral litmus paper on the cut surface. It will become 

 blue. Thus the reaction is alkaline. Stimulate a muscle until it 

 ceases to respond and repeat the test. It will be acid, turning the 

 paper red. A more elegant form of the experiment is to inject 

 under the skin of the back of a frog a few drops of a strong solution 

 of the dye, acid-fuchsin. This is colourless in alkaline solution, so 

 that when examined next day the muscles have their usual yellow- 

 brown colour, j Excise one and stimulate it until it is fatigued. On 

 cutting across, it will be found to have become red, the change 

 being most obvious when compared with a similar muscle which 

 has remained at rest. 



