THE MECHANICAL CHANGES OF MUSCLE 



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(6) If we wish to make and break the primary circuit at will by means of a ke, 

 small electro-magnetic signal, interposed in the circuit, is arranged to write on the 

 revolving drum, and so mark the point of stimulation 



FIG. 51. Arrangement of apparatus for recording simple muscle-twitch. 



Iii the figure (Fig. 52) the upper line is the curve drawn by the lever of the muscle 

 as it contracts ; the small upright line shows the point at which the muscle was stimu- 

 lated ; and the second line is the tracing of the chronograph, every vibration representing 

 T J T of a second. 



In the pendulum myograph (Fig. 53) a smoked glass plate is carried on a heavy 

 iron pendulum. At each side the pendulum is armed with a catch, which fits on to 

 other catches at the side of the triangular box, from the apex of which the pendulum 



FIG. 52. Curve of single muscle-twitch taken on a rapidly moving surface . 

 (pendulum myograph). (EO.) 



is suspended. At its lower part the pendulum carries a projecting piece which can 

 knock over the ' kick-over ' key K, thus breaking a circuit hi which is included the 

 primary coil of an induction-coil. The lever attached to the muscle is arranged so as 

 to write lightly on the glass plate. Everything being ready, and the key K closed, the 

 pendulum is raised to A, the catch A is then released, and the pendulum falls at an 

 ever-accelerating rate and then rises again, gradually slowing off- until it is caught 

 again at B. As it passes by the key it breaks the circuit. A break induction shock 

 is sent into the muscle or nerve, which contracts, and a curve is obtained similar to that 

 shown in Fig. 52. Since the rate of the pendulum is constantly varying throughout its 

 course, it is necessary to have a tuning-fork, or time-marker actuated electrically by a 

 tuning-fork, writing just below the muscle-lever. 



In the spring myograph, otherwise known as the trigger or shooter myograph 

 (Fig. 54), a smoked glass plate is also used. " The frame supporting the glass plate 

 slides on two horizontal steel wires. To make the instrument ready for use, the frame 

 is moved to one side, which compresses a short spring. When the catch holding it in 

 this position is released by the trigger, the spring, which only acts for a short space, 



