54 



EXPERIMENTAL PHYSIOLOGY 



Fig. 56. — Trigger Key. 



a wooden, pendulum swinging on two steel points at the upper angle of a large 

 triangular frame. Clips are provided at either side of the frame by which 

 the pendulum may be held at either side. Projecting from the lower end of 

 the middle of the pendulum is a tongue of metal which breaks the contact 

 of a special trigger-key the details of which are shown in fig. 56. It 



consists of a brass trigger d fixed 

 on an axis on which there is a 

 sector j which is in contact with a 

 brass spring a, connected to a 

 binding screw B-. The trigger is 

 represented in the figure in an un- 

 stable position, so that if its upper 

 end is moved slightly to the right, 

 the spring a will bring it into con- 

 tact with the end of the screw s. If, 

 on the other hand, it is moved a little 

 to the left, the spring will make it 

 move downwards. The screw s 

 fits in a pillar p connected by a strip 

 of brass c to a binding screw b 1 . 

 The screw is tipped with platinum 

 and comes into contact with a platinum plate let into the surface of the 

 trigger d. If a current be sent through the key by the binding screws, it 

 is closed only when the trigger d is in contact with the screw s. In that 

 position the trigger is kept in contact by the spring. As soon as d is knocked 

 to the left, the contact is broken and is not again closed if the trigger be carried 

 beyond the neutral point in which it is drawn in the figiue, being prevented 

 from returning by the spring a. 



Experiment 8. — Take a simple muscle curve with this myograph. In- 

 sert the trigger key in the circuit of , the primary coil, and place a Du Bois 

 key in the secondary. Make a hyoglossus preparation and fix it to the 

 recording lever, with electrodes for stimulating it directly. Close the trigger 

 key, and with the pendulum hanging vertically bring the writing point to 

 the surface ; next bring the contact at the bottom of the pendulum to touch 

 the trigger key while this is still pressed tightly to the screw s. Make a 

 little vertical mark with the writing lever at this spot. This marks the 

 point of stimulation. Now lift the pendulum until it is caught in the catch 

 on the right, and having opened the Du Bois key, release the pendulum, 

 which will then swing across and be caught in the catch on the opposite side. 

 At the middle of the swing it will knock over the trigger, and the break shock 

 in the secondary will thus excite the muscle. Finally draw a zero abscissa 

 line moving the pendulum by hand, and take a time tracing underneath. 



Fig. 57 shows the form of curve thus obtained. The zero abscissa 

 line is, of course, an arc of a circle of radius equal to the length of the 

 pendulum. 



The form of the curve is identical with those already studied in 

 previous experiments. Time measurements should be made of the 

 curve. 



