28 PRACTICAL PHYSIOLOGY 



Again, when the muscle does begin to pull on the lever, it will do so 

 with a sudden jerk, which may cause a light lever to fly up out of 

 control of the contracting muscle ; this, again, makes the muscle 

 appear to have undergone greater shortening than it really has (see, 

 however, Chapter XIII.). Further, the relaxation of a muscle being 

 purely passive, the period of relaxation of an insufficiently weighted 

 muscle is much prolonged, and the writing may fail to reach the base- 

 line again. 1 



In order to get over these instrumental difficulties, the muscle-lever 

 must be as light as is consistent with rigidity, and the muscle must be 

 suitably loaded, the weight being attached near the axis of the lever 

 for the following reasons : the nearer it is to the axis, the less move- 

 ment will it undergo, and therefore the less will be its inertia of move- 

 ment and the more uniform the tension on the muscle throughout the 

 curve. This disposition of the weight also helps to reduce the after- 

 vibrations or ' shatter '-curves which are frequently seen following the 

 relaxation (Fig. 34). Compare with this Fig. 32 taken from the 

 same muscle ; by hanging a weight of 30 grams near the axis of 

 the lever the shatter curves have been nearly eliminated, and are 

 represented by the slight oscillation between the two vertical lines at 

 the end of the curve. 



It may be pointed out that in the living body the muscles are 

 always weighted when they contract, and even when relaxed they are 

 under considerable tension ; for they are really shorter than the distance 

 between their points of origin and insertion, and their antagonists are 

 always exerting a certain pull on them, and some muscles, such as 

 the deltoid, are considerably stretched by the weight of a limb. 



The length of the lever is of some importance ; for, besides the fact 

 that length reduces the rigidity of a light lever, a further deformation 

 of the curve is introduced by increasing the magnification. As the 

 writing point is raised, it tends to leave the drum, and in the course of 

 a much magnified curve is only kept on the drum by the lengthening 

 out of the spring formed by the writing point. Therefore the more 

 the writing point is raised above the horizontal, the more the magnifi- 

 cation is constantly increasing. For this reason the muscular move 

 ment should not be magnified more than is sufficient to make the 

 record of it clear. 



Although muscle curves, as accurate records of the muscular move- 



1 Muscles during the cold of winter, even when properly weighted, frequently 

 show this 'contraction-remainder.' If cold be the cause, turnback the 'trouser* 

 of skin and pour over the muscle some normal tap-water saline heated in a test 

 tube to 25 C. Cf. footnote on p. 33. 



