THE IRRITABILITY OF NERVE 523 



of cold-blooded animals, when isolated from the body retain their living 

 attributes for several hours under the ordinary conditions which can be 

 readily supplied in the laboratory. They, therefore, serve as specially 

 favorable experimental material. 



The muscles of frogs, turtles, and other cold-blooded animals illustrate 

 practically all the facts which can be shown by the muscles of warm-blooded 

 animals and are therefore most advantageously used. 



1. The Muscle-nerve Preparation. The classical muscle-nerve prep- 

 aration is the gastrocnemius muscle and the sciatic nerve. Prepare it 

 as follows: Kill the frog by pithing. This is done by grasping the frog 

 firmly in one hand and w r ith the other making a cut with a blunt scalpel 

 through the cranium just over the medulla, turning the scalpel so as com- 

 pletely to destroy the medulla. Now take a slender knitting needle, quickly 

 run it up into the cranial cavity to destroy the brain, and down the spinal 

 canal to destroy the cord. After a brief spasmodic contraction of the muscles 

 of practically the entire body, the frog remains limp and motionless. In 

 making the muscle-nerve preparation it is better to isolate the tendon first, 

 then the nerve, and finally the femur. The nerve should be prepared as 

 long as possible and should not be allowed to come in contact with the skin. 

 If the preparation is to be used in a moist chamber, the skin should be entirely 

 removed; if it is to be used in the open air, the skin should be left on. Use 

 care not to stretch the nerve, and protect it from evaporation. 



2. The Irritability of Nerve. j Prepare a muscle nerve with its skin 

 on and do not cut away the foot. Mount it by inserting the femur in a 

 muscle clamp, letting the leg extend vertically upward, and the foot hang 

 over. The nerve should lie along the exposed moist femur, one end being 

 slightly free. Stimulate the nerve in the following ways: 



a. Electrical Stimuli. Apply the electrodes from the secondary coil of an 

 induction apparatus to the tip of the nerve. When an induction current of 

 sufficient strength is produced, the muscle to which the nerve is attached will 

 give contractions, thus moving the foot. Notice that contractions occur with 

 both make and break induction. Apply the electrodes from the two poles of 

 a 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 



