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MUSCLE-NERVE PHYSIOLOGY 



then to the primary coil c, and to wires covering two upright pillars of soft iron, b, to the 

 pillar a, and out by the wires to the battery. In passing along the wire b the soft iron is 

 converted into a magnet, and so attracts the hammer, /, of the spring, breaks the connection 

 of the spring with d f , and so cuts off the current from the primary coil, and also from the 

 electro-magnet. As the pillars, b, are no longer magnetized the spring is released, and the 

 current passes in the first direction, and is in like manner interrupted. At each make and 

 break of the primary current, currents corresponding are induced in the secondary coil. 

 These currents are opposite in direction, but are not equal in intensity, the break shock 

 being greater. In order that the shocks should be nearly equal at the make and break, 

 a wire, figure 318, e, connects e and d', and the screw d' is raised out of reach of the spring, 

 and d is raised as in figure 318, so that part of the current always passes through the 

 primary coil and electro-magnet. When the spring touches d the current in b is diminished, 



FIG. 318. Diagram of the Course of the Current in the Magnetic Interrupter of Du Bois 

 Reymond's Induction Coil. (Helmholz's modification.) 



but never entirely withdrawn, and the primary current is altered in intensity at each con- 

 tact of the spring with d, but never entirely broken. 



Preparation of a Muscle for Contraction under Stimuli. The muscles of the frog 

 are most convenient for the purpose of recording contractions. The frog is pithed, that 

 is to say, its central nervous system is entirely destroyed by the insertion of a stout needle 

 into the spinal cord, and the parts above it. One of its lower extremities is used in the 

 following manner. The large trunk of the sciatic nerve is dissected out at the back of the 

 thigh, and a pair of electrodes is inserted behind it. The tendo Achillis is divided from 

 its attachment to the os calcis, and a ligature tightly tied round it. This is the tendon 

 of the gastrocnemius, which arises from above the condyles of the femur. The femur is 

 now fixed to a board covered with cork, and the ligature attached to the tendon is tied to 

 the upright of the muscle lever, figure 319, B. When the muscle contracts the lever is 

 raised. It is necessary to attach a small weight to the lever. In this arrangement the 

 muscle is in situ, and the nerve disturbed from its relations as little as possible. 



The muscle may, however, be detached from the body with the lower end of the femur 

 from which it arises, and the nerve going to it may be taken away with it. The femur 

 should be divided at about the lower third, and the bone fixed in a firm clamp ; the nerve 

 is placed upon two electrodes connected with an induction apparatus, and the lower end 

 of the muscle is connected by its tendon with a lever which can write on a recording 

 apparatus. 



To prevent evaporation this so-called muscle-nerve preparation is placed under a glass 

 cover (moist chamber, figure 350). The air in the moist chamber is kept moist by means 

 of water adherent to its sides. 



Recording the Effects of a Single Induction Shock. With a muscle-nerve preparation 

 arranged in either of the above ways, on closing or opening the key in the primary circuit 

 we obtain and can record a contraction, and if we use the clock-work apparatus revolving 

 rapidly, a curve is traced such as is shown in figure 320. 



