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 conveniently fast 

 speed. The glazed paper is blackened by holding a gas flame contain- 

 ing 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 (myograph 

 lever), which may be straight (Fig. 33), but which may also conveniently 

 take the crank form (Fig. 34). In this case the fulcrum 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; 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-gramme 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. The muscle is 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 freely stretched, and 

 only become so after the muscle has begun to contract ; the muscle is 

 then described as after-loaded. 



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. 



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 



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