62 PRACTICAL PHYSIOLOGY 



also seen that as a muscle shortens its absolute contractile force 

 decreases ; therefore, it is clear that the after-load should be thrown 

 on to the muscle at the instant of contraction, when the contractile 

 force of the muscle is at its maximum, and not later; this is the 

 arrangement in the body. Further, it would be an advantage if the 

 load decreased as the contractile force of the muscle during its con- 

 traction decreased; this is not usually the case in the body, but it 

 does occur in certain movements, as, for instance, in jumping or 

 when, with the upper arm horizontal, a weight in the hand is raised 

 by flexing the forearm on the elbow. 



CHAPTER XIV. (Advanced). 

 SUMMATION OF STIMULI. 



IN a previous chapter the subject of summation of contractions has been 

 dealt with. This summation of 'effect' must be distinguished from 

 the summation of stimuli, by which an inadequate stimulus, if repeated 

 sufficiently often, becomes first adequate and then for a time increas- 

 ingly 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 simple 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 5 seconds. 



