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. This 

 contraction is effected by a change of form. 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 contrac- 

 tion 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 in fact use the 

 graphic method either for registering the changes in volume or for 

 registering changes in tension of a muscle which is prevented from 

 contracting. 



In order to record the muscle -twitch on 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 inser- 

 tion 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 



217 



