16 EXPERIMENTAL PHYSIOLOGY. 



stretching, pressing, drying, or chemical agents. The loss of 

 water may be prevented by keeping the muscle in air saturated 

 with water vapour or by frequently applying water to the 

 exposed surface. But bathing the muscle with water produces 

 consequences as serious, perhaps, as drying, because salts and 

 other substances are drawn out of the cells by osmosis. Now 

 if we add the same salts in the same percentage as occurs in 

 the tissue to the water which is used to bathe the muscle 

 such a solution can be used to prevent evaporation without 

 taking salts from the cells. NaCl dissolved in water to the 

 extent of 0.7% produces an isotonic solution somewhat similar 

 to the tissue fluid of the frog, and is therefore useful for keeping 

 the tissues moist without the injurious effect of pure water. 

 Apply this solution to both the nerve and muscle frequently. 



NERVE-MUSCLE PREPARATION. 



Experiment 3. Different Kinds of Stimuli. Fix the femur 

 of a nerve-muscle preparation in the flat-jawed clamp fastened 

 to a stand. Clamp the nerve holder to the stand just below 

 and then lay the whole sciatic nerve on the glass plate. Fasten 

 the muscle lever underneath. Attach the tendon, by means 

 of a hooked pin to the lever. Now adjust the after-loading 

 screw so that the muscle supports no weight but is fully ex- 

 tended when at rest. Place a ten gram weight upon the scale 

 pan (Fig. 2). In practically every experiment in muscle-nerve 

 physiology, the muscle is after-loaded in this fashion. This 

 after loading keeps the muscle extended and takes the slack 

 so that when contraction occurs there will be no sudden jerk 

 which might injure the muscle fibres. Arrange the lever so 

 that it will write on the smoked paper of the kymograph and 

 obtain records with the following types of stimuli : 

 MECHANICAL. Pinch the nerve near its end with forceps. Can 

 you get a response the second time from the same place? 

 CHEMICAL. Place a few salt crystals on the fresh end of the 

 nerve, the injured portion having been cut away. After a few 

 contractions of the muscle thoroughly remove the salt with 

 isotonic salt solution. 



