THE MECHANICAL CHANGES OF MUSCLE 



195 



over a key as it shoots across, and so breaks the primary circuit and excites the nerve 

 or muscle of the preparation. As we know the exact point that the plate reaches 

 when it knocks over the key, we can mark on the contraction curve the exact moment 

 at which stimulation took place. 



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

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

 revolving drum, and so mark the point of stimulation. 



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



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

 t 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 repre- 

 senting ^ 4 of a second. 



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

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



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

 (pendulum myograph). (YEO.) 



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

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

 knock over the * kick-over ' key K, thus breaking a circuit in 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 

 i;} 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 

 sides 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, 



