The Muscular System. 



249 



the fourth stage consists of one or two small curves, due to 

 the elasticity of the muscle, termed the stage of elastic 

 vibration. All these periods are seen in the curve in Fig. 21. 

 As the muscle becomes fatigued the latent period be- 

 comes longer, and the contraction is slower and shorter ; 

 fatigue diminishes the elasticity of muscle. If a suc- 

 cession of shocks be passed into the muscle in sufficiently 

 rapid sequence tetanus is produced. On the recording 

 apparatus there is a long sharp upstroke, representing the 

 second stage ; the tetanic period is represented by a long 

 irregular line, and when the tetanus passes off there is a 

 sudden relaxation representing the third stage (Fig. 22). 



Yx G . 22. — Tetanus produced with the ordinary Magnetic 

 Interrupter of an Induction Machine (recording surface 

 travelling slowly). 



The interrupted current is thrown in at a (Foster). 



The irregular line shows that tetanus consists of a series of 

 short contractions, with an insufficient interval for complete 

 relaxation— in other words, it is not a single long contrac- 

 tion. A muscle cannot continue permanently in a tetanic 

 condition, fatigue occurs followed by relaxation. 



Besides electrical stimuli, muscles may be excited by 

 mechanical stimuli — such as a prick or tap — or the appli- 

 cation of acids or metals to the muscle, some of which act 

 directly on the muscle substance, others through the 

 medium of the nerve. 



Electrical or other stimulation of involuntary muscle 

 behaves much the same as voluntary, excepting that the 



