ADVANCED EXPERIMENTAL PHYSIOLOGY 



287 



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 condition 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. 



Progressive After-loading of a Muscle. With the same arrangement 

 of apparatus as in the preceding experiment, record a single con- 



FIG. 171. Effect of progressive after-loading of a gastrocnemius. Actual load on 

 muscle, 4 grms. Magnification, 5. Temp., 10 C. (A.P.B.) 



traction of the muscle when just after-loaded, draw a base line and 

 mark the point of stimulation. Now raise the after-loading screw 

 until the writing point is on a level with the highest point of the 

 preceding curve ; draw a fresh abscissa at this level and record a 

 contraction; the point of stimulation will be the same as before. 

 Repeat this process until the muscle can no longer lift the lever off 

 the after-loading screw (Fig. 171). 



From this experiment we see that, in a series of contractions each 

 more after-loaded than the last, a muscle is able to undergo a little 

 further shortening each time until it reaches its maximal shortening ; 

 for the explanation of this see page 294. Also by measuring the 

 heights of the contractions above their respective abscissae, we learn 

 that the longer after stimulation it is before the muscle meets the 

 resistance of a given weight, the less is the muscle then able to 

 overcome that resistance and raise the weight. In other words, as 

 a muscle contracts its extensibility progressively ^increases, and its 

 absolute contractile force decreases, until at the height of its con- 

 traction its extensibility is greatest and its absolute contractile force 



