47 6 



MUSCLE-NERVE PHYSIOLOGY 



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, but never entirely 

 withdrawn, and the primary current is altered in intensity at each contact of 

 the spring with d, but never entirely broken. 



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

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



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 attach- 

 ment to the os calcis and a ligature tightly tied around 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 

 is 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 3 50) . The air in the moist chamber 

 is kept moist by means of water adherent to its sides. 



