THE MUSCULAR SYSTEM 68 



reaches a muscle, the numerous branches of the nerve 

 one for each individual muscle cell distribute the 

 stimulus to all parts at practically the same instant. 

 The end plates are situated in each muscle fiber near 

 the middle between the two ends, and the fiber begins 

 there to contract. The two ends draw toward each 

 other ; the fiber becomes swollen and shorter. In the 

 muscle as a whole contraction is simply the sum of the con- 

 tractions of all the minute fibers which compose it. In 

 the muscle fiber contraction appears to be some compli- 

 cated movement of the molecules which produces a change 

 in the appearance of the stripes. It is found that in 

 those places where swift and rapid contraction is called 

 for, the muscular tissue has almost invariably the striped 

 fibers. 



74. When a muscle is made to contract by a single 

 electric shock, or by other artificial means, the movement 

 is sudden and brief. Voluntary muscle, however, under 

 its natural nervous stimulus, never contracts with a 

 twitch. Its action is rather that of continued gentle 

 vibration, called tetanus, such as follows a rapid series 

 of electric shocks which leave no time for relaxation. 

 The nervous stimulus comes to the muscle in a quick 

 succession of impulses, about twenty in a second, so 

 that one vibration is succeeded by a second before the 

 first has ceased to agitate the muscle cell. 



75. Internal Changes in Muscle under Stimulus. Some of 

 the energy set free by a contracting muscle appears as work 

 done, weight lifted, etc., while a considerable amount 

 becomes heat, for the temperature of muscle always rises 

 under contraction. Certain chemical changes also ap- 

 pear. Variable amounts of carbonic acid and lactic acid 

 are set free, and oxygen is used up. Electric changes 



