SECTION III 



THE MECHANICAL CHANGES THAT A MUSCLE 

 UNDERGOES WHEN IT CONTRACTS 



IF a skeletal muscle, such as the gastrocnemius, be stimulated either directly 

 or by the intermediation of its nerve by any of the means mentioned in the 

 foregoing chapter, it responds by a single short sharp contraction, followed 

 immediately by a relaxation. The volume of the muscle does not alter in the 

 slightest degree, but each muscle-fibre and the whole muscle become shorter 

 and thicker. At the same time, if a weight be tied on to the tendon of the 

 muscle, the muscle during contraction may raise the weight and thus perform 

 mechanical work. In order to determine the time relations of the simple 

 muscle contraction or the muscle-twitch, and to study its conditions, it is 

 necessary to employ the graphic method, so as to obtain a record of the 

 changes in shape of the muscle during contraction. We may use the graphic 

 method either for recording the changes in shape or for registering changes in 

 tension of a muscle which is prevented from contracting. 



In order to record the contraction of the frog's gastrocnemius, the muscle is excised 

 together with a portion of the femur to which it is attached, and the whole length 

 of the sciatic nerve from its origin in the spinal canal to its insertion into the muscle. 

 The femur, to which the gastrocnemius is attached, is clamped firmly, and the 

 tendo Achillis attached by a thread to a light lever, free to move round an axis at one 

 end. The point of this lever is armed with a bristle (anything that is stiff and pointed 

 will do), which just touches the blackened surface of a piece of glazed paper. This 

 paper is stretched round a cylinder (drum) which can be made to rotate at any constant 

 speed required. If the drum is moving, the point of the bristle draws a horizontal 

 white line on the smoked paper. 



If a single induction shock be sent through the nerve of the preparation the lever 

 is jerked up, falling again almost directly, and a curve is drawn like that shown in 

 Fig. 52. A similar curve is obtained if the muscle be stimulated directly. 



In all such graphic records we should have also 



(1 ) A time record. This is furnished by means of a small electro -magnet, armed with 

 a pointed lever writing on the smoked surface. This electro -magnet (time marker or 

 signal) is made to vibrate 100 times a second (more or less as may be required) by 

 putting it in a circuit which is made and broken 100 times a second by means of a 

 tuning-fork vibrating at that rate. The tuning-fork 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 arranged that it knocks 



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