THE ACTION CURRENT OF MUSCLE 567 



chamber with a long nerve and nonpolarizable electrodes. Use a small 

 platinum stimulating electrode arranged to stimulate the nerve at differ- 

 ent points, a, on the muscle side of the boot electrode, b, next to the muscle 

 but as close to the boot electrode as possible. Use single induction cur- 

 rents for stimulation of the nerve during the flow of a galvanic current as 

 in Experiment 17. 



Record and check the change in nerve irritability during electrotonus 

 against the diagram, figure 335; 



1 8. The Action Current of Muscle, a. The Rheoscopic Frog. The 

 action current produced by a single contraction of a gastrocnemius muscle 

 is enough to stimulate the sciatic nerve of a second preparation when 

 the second nerve is laid across the first muscle in such a way as to be a 

 conductor of its action current. Expose the two gastrocnemius muscles 

 of a frog pinned horizontally to a frog board. These need not be separated 

 from the skeleton. Bring the two near to each other and pin the knees 

 to the frog board. Attach the tendon of the second to a lever and bring 

 its long sciatic nerve in contact with the tendon and the belly of the first 

 muscle. Now stimulate the nerve of the first muscle. When the first 

 muscle contracts in response to the nerve impulse from its nerve, the 

 action current developed will flow through the loop of the second nerve in 

 physical contact only with the first muscle. Stimulation of the nerve is 

 indicated by the contraction of the second muscle as recorded on the 

 drum. 



b. By careful manipulation the "current of injury" or "demarcation 

 current" which develops in the first muscle when cut across the middle 

 will also stimulate the second nerve. Use a glass rod to adjust the nerve. 

 Bring the middle of the nerve in contact with the proximal tendon of the 

 cut muscle, then suddenly drop the free end of the nerve onto the cut end of 

 the muscle. The nerve will be stimulated and the second muscle will 

 contract. 



c. Action Current of the Heart. Prepare a perfectly fresh gastrocnemius 

 with a long sciatic. Expose a large turtle's heart. Support the sciatic 

 by a glass rod so that it touches the heart at the base and the apex only. 

 The cardiac action current will stimulate the sciatic at each contraction. 



19. Unipolar Stimulation. The stimulating effect of an electric cur- 

 rent is proportional to the intensity of the current per unit area of the 

 physiological electrode. When one electrode is made indifferent by spread- 

 ing the current over a large surface, then the stimulations will be limited 

 to the smaller electrode. Use the Harvard apparatus, bind the indifferent 

 electrode on the neck and with the other verify the motor points in the 

 arm shown in figure 336. 



This method can be used to demonstrate DuBois-Reymond's law on 

 human nerves. Apply the stimulating electrode over the median nerve 

 at the elbow. Stimulation with a medium current will occur only on clos- 



