1 10 IRRITABILITY AND CONTRACTILITY. [CH. nil. 



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 evident ; 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. Thermal ; for instance touching the nerve or muscle with 

 a hot wire. 



4. Electrical ; the constant or the induced current may be 

 used. 



In all cases the result of the stimulation is a muscular 

 contraction. 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 versd ; thus glycerine 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 disturb- 

 ances ; and moreover, the effect of the stimulus is propagated 

 along the nerve or muscle without loss. 



