MUSCLE CURRENTS 147 



electro-positive to one more injured or more active respectively. 

 Perfectly uninjured and resting muscle cannot be experimented upon 

 because simple exposure of tbe muscle causes sufficient injury to set 

 up differences in electro-motivity. Still tbat a perfectly normal 

 resting muscle is iso-electric is practically certain. The greater the 

 care taken in the exposure of the muscle the less are the currents 

 obtainable, and the heart, which can be exposed without injury, 

 shows no current when at rest. Conversely if a muscle be purposely 

 injured it yields a current which is in proportion to the injury ; and 

 an injured resting heart also gives a current. 



I. EXPERIMENTS TO SHOW THE EXISTENCE OF THE 

 VARIOUS CURRENTS BY PHYSIOLOGICAL MEANS 



Experiment 1. — Galvani's experiment of contraction with metals. Solder 

 a piece of copper wire to the end of a piece of zinc wire. The wires shoidd 

 be fairly'thick and about 6 cm. in length. They may then be bent round into 

 the form of a U. Pith a frog, remove the skin from the back of the thigh and 

 dissect out the sciatic. Lift up the nerve on the end of one of the wires, and 

 with the end of the other touch any part of the frog. At each contact the 

 muscles of the leg give a twitch. In the more classical form of the experi- 

 ment the vertebral column is cut across just above the sacrum, the whole 

 of the abdominal viscera removed and the skin stripped off the legs. The 

 urostyle and the muscles attaching it to the ilia are removed so as to 

 expose both sciatic plexuses. An s-shaped hook of copper wire is then passed 

 round both plexuses, and by this the frog is suspended to a clean iron tripod. 

 If the tripod be now tilted so that any part of the legs of the frog touches 

 the metal a twitch occurs in both legs. 



This was Galvani's first experiment, which he brought forward to 

 prove the existence of animal electricity, and which led to the cele- 

 brated controversy between him and Volta. Volta proved — by the 

 invention of the voltaic pile — that the contraction was in reality due 

 to the current caused by contract between two dissimilar metals. 



Experiment 2. — Galvani's experiment of contraction without metals. Very 

 carefully dissect out a sciatic nerve and gastrocnemius and place it upon a 

 clean dry glass plate. Lift up the nerve on a glass rod drawn out into the 

 form of a hook and lower the cut end on to the lower end of the muscle. 

 With each contact the muscle contracts in response to the stimulus started 

 in the nerve by the closure of the current of injury of the muscle. If the ex- 

 periment does not at once succeed, injure the lower end of the muscle by 

 touching it with a hot glass rod or wire and then repeat the experiment. 



This is the crucial experiment which definitely proved that part of 

 Galvani's views as to the existence of animal currents. 



Experiment 3. — Kuhne's experiment of contraction without metals. Place 

 a clean glass plate so that it projects about one inch over the edge of the 

 table. Make two rolls of china clay moistened with normal saline and place 

 then parallel to each other, so that they project beyond the edge of the 

 glass plate, and turn down the projecting ends till they hang below the 



l2 



