COMPARISON or MUSCLE AND STEAM ENGINE 281 



this result that the nerves are especially well adapted for 

 the carriage of impulses to the fibers, though they are 

 not the sole means of stimulating the fibers. The sudden 

 contraction of a muscle when struck is thus explained as 

 merely a response of the cells to another form of stimu- 

 lus. The nerves, however, constitute the normal means 

 of transmitting an impulse to the muscle from out- 

 side. 



Method of muscle contraction under stimulus. — If, in 

 place of a simple apparatus, such as that described in the 

 preceding paragraph, we substitute an instrument which 

 can apply a rapid succession of shocks to the nerve end, 

 it is possible to make the stimuli so rapid that the second 

 contraction will begin before the first relaxation has ended. 

 Under these conditions the muscle rapidly shortens until 

 it finally reaches a point where it can contract no more. 

 Successive stimuli will, then, merely serve to maintain 

 it in a contracted state until finally the contractile power 

 becomes exhausted, and extension occurs in spite of stim- 

 ulation. Such a progressive series of contractions are 

 spoken of collectively as a tetanic contraction. It is 

 supposed that in the living body all nerve impulses are 

 of this tetanic character, as the time of a simple contrac- 

 tion is only al)Out a tenth of a second, and very few move- 

 ments are of so short duration. 



Comparison of Mttscle axd Steam Engine as 

 Energy Producers. 



The substance in the muscle cells has the power of irri- 

 tability, and the nerves are the natural conductors of the 

 stimuli which excite this irritability. What happens iii 

 a muscle fiber or cell when this stimulus is received? We 



