90 THE NEKVnrS SYSTKM AND ITS CONSERVATION 



conception. Though it >eems at first to be rather difficult 

 tn entertain, \ve ^hall find that a strong ea-e can he made 

 for it. 



If every muscle-fiber is comparable with a repeating 

 firearm in which new ammunition is rapidly placed in 

 position to be discharged, it is hard to see how a stimulus 

 strong enough to have any effect can do less than discharge 

 all the energy which can possibly be released at the 

 moment. If a blast is to be fired, it is immaterial whether 

 a small or a large spark is used for the purpose. This is 

 precisely the case with the heart. The form of muscular 

 tissue composing the walls of this organ is said to obey 

 the "all-or-none law," by which we mean that none but 

 full-sized contractions can be obtained from it. 



It is the conception of Lucas that the isolated fiber of 

 skeletal muscle would be found to conform to the all-or- 

 none principle if we could make experiments upon it. 

 The real difference between such fibers and those of the 

 heart, he supposes, lies in the insulated character of the 

 former and the protoplasmic union of the latter. A 

 process of contraction once started in the heart will 

 usually sweep over all the bundles which make up its 

 substance. In skeletal muscle such a process is definitely 

 limited to the units actually reached by nerve-impul>es, 

 and a maximal response cannot be secured unless the 

 stimulation can be made to affect every fiber through its 

 end-plate. A stimulation of only a certain proportion 

 of the units will result in a movement or a tension at 

 the tendon le<s than the maximum. Because the number of 

 unit- i- large th- number of possible gradations bet ween 

 the lea-t and the greatest will he correspondingly large. 



The idea set forlh by Lucas can be exprosed in other 

 words. It makes what we call a muscle assuming unity 

 of action for all I he fibers which can act upon a common 

 tendon a imisciilulinr, that is, a system capable of more 

 or less locali/ed action at some times and of a mutual 

 reinforcement among all it- part- when all its powers are 

 called in play. When we venture to speak of a muscle 



