CH. VIII.] VARIETIES OF STIMULI 85 



The first thing to do is to make from a pithed frog a muscle-nerve 

 preparation ; the muscle usually selected is the gastrocnemius, the 

 large muscle of the calf of the leg, with the sciatic nerve attached. 

 For some experiments the sartorius or gracilis may be used; but 

 nearly all can be demonstrated on the gastrocnemius. 



The tendon of the gastrocnemius may be tied to a lever with a 

 flag at the end of it, and thus its 

 contractions rendered more evi- 

 dent; the bone at the other end 

 is fixed in a clamp. Stimuli may 

 be applied either to the nerve or 

 to the muscle. If the stimulus is 

 applied to the nerve, it is called 

 indirect stimulation ; the stimulus 

 starts a nervous impulse which 

 travels to the muscle ; the muscle 

 is thus stimulated as it is in voluntary contraction by a nervous 

 impulse. Stimulation of the muscle itself is called direct stimulation. 

 These stimuli may be : 



1. Mechanical ; for instance a pinch or blow. 



2. Chemical ; for instance salt or acid sprinkled on the nerve or 

 muscle. 



3. Electrical ; the constant or the induced current may be used. 



In all cases the result of the stimulation is muscular contrac- 

 tion. Of all methods of artificial stimulation, the electrical is the 

 one most generally employed, because it is more under control 

 and the strength and duration of the stimuli (shocks) can be 

 regulated easily. We shall therefore have to study some electrical 

 apparatus. 



Chemical stimuli are peculiar, for some which affect muscle do 

 not affect nerve, and vice versa ; thus glycerin stimulates nerve, but 

 not muscle ; ammonia stimulates muscle, but not motor nerves. 



We may regard stimuli as liberators of energy ; muscle and nerve 

 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. 



