ELECTRICAL EXCITATION. 125 



7. Excitation by Interruption of the Direct or Battery 



Current. For this purpose it is necessary to arrange the circuit so that its 

 intensity can be varied at will. This might be accomplished by the inter- 

 position of large resistances, but such a method would be so inconvenient as 

 to be impracticable. The method always used is to connect the poles of the 

 battery by a side wire, whose resistance can be varied at pleasure. As the 

 resistance of nerve and muscle is very high, the strength of the current in 

 the circuit varies approximately inversely as the resistance of the side wire. 

 A graduated side wire suitable for this purpose is called a rheochord. The 

 rheochord commonly used is that of du Bois-Reymond ("Handbook," 

 fig. 298). Connect the two terminal binding screws of the rheochord with 

 the battery (a single Daniell's cell) interposing a key ; connect with the 

 same binding screws the two end screws of the reverser ("Handbook," 

 figs. 299, 506), and finally insert the wires from the electrodes in the two 

 central screws (i & 2). Prepare the sacral plexus and rheoscopic limbs as 

 before, and arrange the electrodes. Diminish the resistance of the rheochord 

 to the utmost, and observe that on opening and closing the circuit, no 

 contraction takes place. Then gradually increase the resistance. At first 

 the muscles respond only to closure, subsequently to "make" and "break," 

 whatever the direction of the current. On continuing the observation, par- 

 ticularly with stronger currents, it will be observed that the "make" and 

 " break " effects are in no instance equal, and that the nature of the inequality 

 is influenced by the direction of the current. The results are further modified 

 by exhaustion or injury of the nerve. 



8. Excitation of a Motor Nerve by contact with a Con- 

 tracting Muscle. The Secondary Twitch. After preparing the 

 sciatic nerve as above directed, expose the gastrocnemius as directed in 6. 

 Seize its tendon with the forceps and separate it from its attachments. Cut off 

 the tibia and femur, close to the knee on either side, along with the muscles 

 and other soft parts, taking care not to injure the nerve. A gastrocnemius 

 with its nerve as described, constitute a "nerve muscle preparation." Two 

 such preparations are required. 



Place one of them, b, on a glass plate, and fix the other, a, along the edge 

 of a small piece of board. Then place the board on the glass plate in such a 

 position that the nerve of b can can be readily laid on the muscle of a. 

 Excite by a single induction shock passed through its nerve. At the same 

 moment that a contracts, b will contract. Then repeat the experiment, but 

 instead of passing single induction shocks, faradize the nerve. Tetanus is 

 produced in b, which lasts so long as a is tetanized. Ascertain that the effect 

 is not due to escape of current, by ligaturing the nerve and repeating the 

 experiment. 



9- Mechanical Excitation. Mechanical Tetanus. Connect a Grove 

 cell with the " Tetanometor," introducing a key into the circuit. The wire 

 from the zinc terminal of the battery must be inserted in the binding screw 

 marked Z, that from the platinum in K. Adjust the apparatus so that on 

 closing the key the ivory hammer vibrates so as to excite, without destroying, 

 a nerve placed on the ivory groove. The effect produced is identical with 

 tetanus by faradization. 



