THE RECORDING OF MUSCULAR CONTRACTIONS 



27 



kept stretched by the weight, so that the connecting thread is taut. 

 Under these circumstances the muscle is said to be free-weighted. 



There should always be a screw near the fulcrum which is intended to support 

 the lever in certain experiments. The screw can be adjusted so that the muscle 

 and thread are not fully stretched, and only become so after the muscle has 

 begun to contract ; the muscle is then described as after-loaded. 



The simple muscle-curve. Arrange the drum in the primary cir- 

 cuit of the induction coil (in the manner shown in fig. 33), so that, as 

 the drum revolves, a pin (c) which projects from it, by just touching a 



PIG. 32. MUSCLE-NERVE PREPARATION SUSPENDED AND ATTACHED TO A STRAIGHT 

 MYOGRAPH LEVER, a. AFTER-LOADING SCREW ; 6, ELECTRODES. 



needle fixed outside, instantaneously makes and breaks the circuit at 

 each revolution. A pair of electrodes is either brought in contact with 

 the muscle itself or the nerve (ri) is laid upon the electrodes, which are 

 connected through a short-circuit key with the secondary coil. The 

 secondary coil is placed at such a distance from the primary that the 

 " make " shock is insufficiently strong to act ; the " break " shock 

 therefore alone furnishes the stimulus. 1 Before the lever is allowed 

 actually to touch the cylinder, determine that the apparatus is all in 

 working order, and at what distance of the secondary from the primary 



1 It is possible to employ a single induction shock as the stimulus by intro- 

 ducing a break key into the primary circuit and making the pin open this key 

 as the drum revolves. 



