IRRITABILITY OF MUSCLE 493 



dry battery. When the current of the battery is established a contraction will 

 occur, but does not continue during the time of the flow of the current. When 

 the current is stopped a second contraction occurs. The nerve is irritable to 

 both galvanic and faradic currents. 



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 fiber 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: 



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

 above, applying the stimuli, electricity, etc., directly to the muscle substance, 

 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 sub- 

 stance, 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. 



4. Independent Irritability of Muscle. The influence of curara on 

 the muscle-nerve preparation is demonstrated as follows : Destroy the brain 

 only of a frog by pithing, taking care not to injure the blood-vessels of the 

 spinal canal. Place the animal on a glass plate with its back up and dissect out 

 the sciatic nerve. Use care not to injure in any way the accompanying femoral 

 artery. Pass a ligature of linen thread under the sciatic and around the mus- 

 cles and blood-vessels of the leg so as completely to shut off the circulation on 



