102 



PRACTICAL PHYSIOLOGY 



arrangement depends upon the increased extensibility of contracting 

 muscle ; for in this way liability to rupture is reduced ; further, 

 there is a saving of energy in pulling at a dead weight through an 

 elastic spring, instead of through an inelastic cord, since some of the 

 energy expended would be lost in a sudden jerk, but, in the case of 

 the spring, is stored up in it and given out again as its elastic recoil. 

 Thus smoothness is imparted to even the most sudden movements. 



CHAPTER VI 

 SUMMATION OF STIMULI 



In a previous chapter the subject of summation of contractions 

 has been dealt with. This summation of " effect " must be dis- 

 tinguished from the summation of stimuli, by which an inadequate 

 stimulus, if repeated sufficiently often, becomes first adequate and 

 then for a time increasingly effective. This is a summation of 

 " cause," and probably plays an important part in the life of all 

 living matter. 



In order to demonstrate the summation of stimuli, arrange the 

 apparatus for stimulating a gastrocnemius muscle directly with 

 single induction- shocks, using a simply key in the primary circuit. 

 Place the secondary coil at such a distance from the primary that 

 the break-shocks are just subminimal. Repeat the stimulus every 



five seconds. It will be 

 found that sooner or 

 later the summed excita- 

 tions will cause a con- 

 traction, and, if the 

 contractions are recorded 

 on a slowly-revolving 

 drum, that a well-marked 

 " staircase " effect is 

 produced (Fig. 103). 



In dealing with the re- 

 sponse of muscle to two 

 successive stimuli, it has 

 been seen that, when 

 the second stimulus falls 

 within the latent period 

 of the first, the muscle is 

 refractory, so far as being 

 able to respond with a second contraction is concerned ; but it is 

 not true that a muscle during its refractory period always entirely 

 ignores a second stimulus. 



In order to investigate this point, the apparatus is arranged as in 

 demonstrating the effect of two successive stimuli (p. 48). The two 

 " strikers " are placed at such an angular distance apart that the 



FIG. 103. Effect of subminimal stimuli 

 repeated every five seconds on gastro- 

 cnemius stimulated directly. 



The dots mark the points at which stimuli were sent 

 in before they became obviously effective. Time 

 marking in seconds. (A.P.B.) 



