100 



PRACTICAL PHYSIOLOGY 



longer tends to raise the lever off the after-loading screw, the muscle 

 is unstretched by any load. Arrange the apparatus so that with 

 the screw in this position the lever is horizontal. Record a single 



contraction of the muscle on a 

 rapidly revolving drum, mark the 

 point of stimulation, and draw an 

 abscissa. Then lower the after- 

 loading screw until the muscle is 

 loaded with the whole weight, and 

 superimpose on the same abscissa 

 and with the same point of 

 stimulation a contraction of the 

 loaded muscle. (Fig. 102.) 



The main differences between 

 these two curves are in the 

 purely after-loaded muscle there 

 is an appreciable lengthening of 

 the latent period owing to the 

 muscle in its unstretched condi- 

 tion having to take in " slack " ; 

 a diminution in the height of 

 the contraction, owing to the 

 absence of tension on the muscle 

 before the contraction began. In 

 other words, moderate initial 

 tension increases the power of a 

 muscle to do work. 



Relation of Load to Work done 

 during Contraction. In order to 

 record the height of contraction 

 for a large range of weights, it 

 is more convenient to record on 

 a stationary drum simply the 

 heights of a series of twitches 

 than to superimpose a large num- 

 ber of curves. The apparatus is 

 arranged for stimulating the 

 muscle with a single maximal in- 

 duction-shock, using a simple key 

 in the primary circuit. A weight 

 is hung near the axis of the lever 

 of such a size that the actual 

 load on the muscle is 50 grms. ; 

 the method of calculating this 

 weight has been already given 

 on p. 99. The muscle is just 



completely after-loaded throughout the experiment in order to get 

 rid of the effect of alterations in the initial tension. With the 

 lever horizontal, the muscle is stimulated, and the height of its 





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