104 IRRITABILITY AND CONTRACTILITY [CH. VIII. 



and other irritable structures undergo disturbances in consequence of 

 a stimulus. The disturbance is some form of movement, visible 

 movement in the case of muscle, molecular movement in the case of 

 nerve. A stimulus may be regarded as added motion. Sir William 

 Gowers compares it to the blow that causes dynamite to explode, or 

 the match applied to a train of gunpowder. A very slight blow will 

 explode a large quantity of dynamite ; a very small spark will fire a 

 long train of gunpowder. So in muscle or nerve the effect is often 

 out of all proportion to the strength of the stimulus ; a light touch 

 on the surface of the body may elicit very forcible nervous and 

 muscular disturbances ; and moreover, the effect of the stimulus is 

 propagated along the nerve or muscle without loss. 



