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HANDBOOK OF PHYSIOLOGY. 



magnetized the cpring is released, and the current passes in the first di- 

 rection, and is in like manner interrupted. At each make and break of 

 the primary current, currents corresponding are induced in the secon- 

 dary 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 (Fig. 286 e') con- 

 nects e and d', and the screw d' is raised out of reach of the spring, and 

 d is raised (as in Fig. 286), 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 d, the current in b is diminished, but never entirely withdrawn, and the 

 primary current is altered in intensity at each contact of the spring with 

 d, but never entirely broken. 



Record of Muscular 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 



FIG. 286. Diagram of the course of the current in the magnetic interrupter of Du Bois Rey- 

 xnoncTs induction coil. (He]mholz 1 s modification.) 



tendo Achillis is divided from its attachment to the os calcis, and a liga- 

 ture is tightly tied round it. This tendon is part of the broad muscle 

 of the thigh, (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 a piece of 

 metal bent at right angles (Fig. 287, B), which ir capable of movement 

 about the pivot at its knee, the horizontal portion carrying a writing 

 lever (myograph). 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 is divided at about the lower third. The 

 bone is held in a firm clamp, the nerve is placed upon two electrodes con- 

 nected with an induction apparatus, and the lower end of the muscle is 

 connected by means of a ligature attached to its tendon with a lever 

 which can write on a recording apparatus. 



To prevent evaporation this so-called nerve-muscle preparation is 



