CHAPTER V 



THE RECORDING OF MUSCULAR CONTRACTIONS 



MUSCULAR contractions are recorded by a lever upon a metal drum 

 covered with highly glazed paper, and caused to revolve by clockwork, 

 or some other form of motor, at a regular rate. With a drum of six 

 inches diameter one revolution in a second is a convenient speed. 

 The glazed paper is blackened by holding a gas flame containing 

 benzole vapour against it while the drum is revolving. The paper 

 must fit evenly and tightly, or it will become burnt. 



The contraction of the muscle is amplified by the lever (myo- 

 graph lever), which may be straight (fig. 32), but which may also 

 conveniently take the crank form (fig. 33). In this case the ful- 

 crum of the lever is at the end of a cork plate, to which the muscle 

 is fastened by a pin passed through the knee-joint, 1 the tendon is 

 attached to the short arm of the lever by means of a thread. The 

 cork plate must be covered with paraffined paper. The lever should 

 be weighted with a 20- or 30-gram weight, attached to it close to 

 the fulcrum, and should be so adjusted as to be nearly horizontal, 

 but with the end a little lower than the fulcrum. 2 The muscle is 



1 In the Keith-Lucas crank myograph the muscle-nerve preparation is 

 enclosed in a vulcanite trough ; the muscle is kept immersed in Ringer's 

 fluid ; the bony attachment of the muscle is fixed by a pin, and electrodes are 

 introduced through holes in the vulcanite. The whole is covered by a glass 

 plate to prevent evaporation. 



2 The following points must be attended to in every graphic record in which 

 a lever is employed: (1) On no account must the lever point be directed 

 obliquely upwards : the result of doing this is to distort the curves which are 

 recorded ; (2) the lever must be directed tangentially to the curve of the drum 

 with the point of the lever slightly curved in towards the surface of the drum ; 



(3) the myograph stand, which carries the cork plate and lever upon a vertical 

 rod capable of being turned on its axis, must always be furnished with a stop, 

 so that the point of the lever can always be brought against the drum with 

 exactly the same amount of pressure as that with which it is originally adjusted ; 



(4) the myograph stand must always be on the right-hand side of the drum, 

 so that the lever extends from right to left, and the drum itself must always be 

 arranged to move in the direction of the hands of a watch, never in the reverse 

 direction. 



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