218 



PHYSIOLOGY 



is maintained in vibration in the same way as the Wagner's hammer of an 

 induction-coil. 



(2) A record of the exact point at which the nerve or muscle is stimulated. This 

 may be obtained in two ways : 



(a) When using the pendulum or trigger myograph, in both of which the 

 recording surface is a smoked flat surface on a glass plate, this latter is so 



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



arranged that it knocks over a key as it shoots across, and so breaks the primary 

 circuit and excites the nerve or muscle cf 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. 



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



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

 (pendulum myograph). (EO.) 



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

 muscle was stimulated ; and the second line is the tracing of the chronograph, 

 every vibration representing ^ 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 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 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 is 



