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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of very rapid and powerful but short-lived contractions start the movement, 

 while the slower red muscles continue it. 



(6) Effect of Tension on the Extent and Course of the Contraction. As we 

 have seen, the rate of the contraction of an ordinary striated muscle is much 

 too rapid to be followed by the eye, and to study the course of the change in 

 form it is necessary to employ some kind of recording mechanism. Every 

 mechanical device for recording the movements of the muscle has inertia, and, 

 if given motion, acquires momentum. Both of these factors would tend to 

 alter the shape of the record, and the more, the greater the weight of the re- 

 cording apparatus. 



A weight, or tension, can be applied to a muscle in various ways, and the 

 form of the contraction will be correspondingly changed. If a muscle is made 

 to work with a considerable weight hanging on it, we speak of it as loaded ; 

 if the weight be connected with the muscle, but so supported that it does 

 not pull on it until the muscle begins to shorten, the muscle is said to be after- 

 loaded; if the weight is the same throughout the contraction, as when the 

 muscle has only to lift a light weight, applied close to the axis of the lever, the 

 contraction is said to be isotonic ; if on the other hand the contracting muscle 

 is made to work against a strong spring, so that it can shorten very little, i. e. 

 has almost the same length throughout the contraction, the contraction is said 



to be isometric. The shape of the 

 myogram recorded as a result of 

 the same stimulus would evidently 

 be very different in these four 

 cases. The effect of a weight to 

 alter the myogram is illustrated in 

 the record given in Figure 40. 

 Increasing the weight prolonged 

 the latent period, and lessened the 

 height and duration of the con- 

 tractions. 



The alterations liable to occur 

 in the form of the myogram as a 

 result of the mechanical condi- 

 tions under which the work is 

 done are 



(1) Prolongation of the latent 

 period. There can be no move- 

 ment of the lever until the inertia 

 of the weight has been overcome, 

 and the first effect of the contrac- 

 tion is to stretch the muscle, a 

 part of the energy of contraction being changed to elastic force, which on the 

 recoil assists in raising the weight. 



(2) Alteration in the shape of the ascending limb of the myograph curve. The 



FIG. 40. Effect of the weight upon the form of the 

 myogram. The gastrocnemius muscle of a frog excited 

 by maximal breaking induction shocks five times, the 

 weight being increased after each contraction, and in the 

 intervals supported at the normal resting length of the 

 muscle; i. e. the muscle was after-loaded: 1, muscle 

 weighted only with very light lever; 2, weight five 

 grams ; 3, ten grams : 4, twenty-five grams ; 5, fifty grams. 

 The perpendicular line marks the moment of excitation. 

 The time is recorded at the bottom of the curve by a 

 chronograph, actuated by a tuning-fork vibrating 50 times 

 per second. 



