286 



PRACTICAL PHYSIOLOGY 



The effect of load, and of its method of application on a single 

 muscular contraction, will be studied in the following ways : (a) the 

 contraction given by a muscle loaded and after-loaded with the same 



weight will be compared ; (b) a con- 

 stant load will be thrown on to a 

 muscle as an after-load later and later 

 in its period of shortening, and the 

 effect on the contractions noted ; (c) the 

 muscle being just completely after- 

 loaded, the height of - contraction, 

 with increasing loads, will be mea- 

 sured and the work done with each 

 calculated. 



Comparison of the Contractions of 

 a Loaded and After-loaded Muscle. 

 Arrange the apparatus for stimulating 

 a muscle with single maximal induc- 

 tion shocks, using the drum as a key 

 in the primary circuit. Fix a gastro- 

 cnemius preparation to a myograph 

 lever, provided with an after-loading 

 p screw ; by raising the screw the metal 

 part of the lever can be supported at 

 any level (Fig. 25). Hang a weight 

 of 50 grms. near the axis and raise the 

 screw until the whole of the weight 

 is just after-loaded ; this point can be 

 ascertained by supporting the weight 

 with the finger, and when the muscle 

 no 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 super-impose on the same abscissa 

 arid with the same point of stimulation a contraction of the loaded 

 muscle (Fig. 170). 



