IRRITABILITY OF MUSCLE 



559 



b. Mechanical Stimuli. Pinch the nerve lightly with forceps, or give it 

 a sudden stroke with the scalpel handle. With each mechanical impact 

 there is a single contraction of the muscle. 



c. Thermal Stimuli. Touch the end of the nerve with a glass rod heated 

 in boiling water. At each time the nerve is brought in contact with the rod 

 there will be muscular contraction, as in the preceding cases. The experi- 

 ment succeeds better if the nerve comes in contact with the rod for several 

 millimeters of length. If the tip of the nerve has ceased to respond, then 

 snip it off with the scissors, and repeat the experiment on the fresh end. 



d. Chemical Stimulation. Many chemical substances when brought in 

 contact with living nerve fibers produce nerve impulses. Try crystals of 

 sodium chloride, magnesium sulphate, dilute ammonia, acetic acid, 10 per 

 cent, nitric acid, i per cent, mercuric chloride. 



Tabulate your observations on all the forms of stimulation used above, 

 by the following outline: 



4. Irritability of Muscle. Repeat the experiments in number 2 

 above, applying the stimuli, electricity, etc., directly to the muscle sub- 

 stance, choosing as far as possible portions of muscle which do not exhibit 

 nerve fiber. The muscle will usually respond by a contraction to each of 

 the above forms of stimulation. 



These tests do not fully demonstrate the direct irritability of muscle 

 substance, since in each case it is possible that nerves may have been 

 stimulated. The nerve influences over the muscle can be eliminated by 

 the use of drugs, as will be shown in the next experiment. 



5. Independent Irritability of Muscle. The influence 01 curara on 

 the muscle-nerve preparation is demonstrated as follows : Ligate one leg of 

 a frog near the thigh to stop the circulation on that side. Now inject 

 under the skin of the back three drops of i per cent, curara, allowing 

 twenty to thirty minutes for absorption. Make the following observations : 

 Place the animal on a glass plate with back up and dissect out the sciatic 

 nerve in each leg. Use care not to injure in any way the accompanying 

 femoral artery. 



a. Stimulate the muscles of the ligatured leg, also the muscles of the 

 curarized leg, each will contract. 



