CH. VI II.] VARIETIES OF STIMULI 103 



centre is started by a sensory impulse from the periphery ; thus 

 when one puts one's hand on something unpleasantly hot, the hand is 

 removed ; the hot object causes a nervous impulse to travel to the 

 brain, and the brain reflects down to the muscles of the hand another 

 impulse by the motor nerves which causes the muscles to contract in 

 such a manner as to move the hand out of the way. 



But the details of muscular contraction can be more readily 

 studied in muscles removed from the body of such an animal as the 

 frog, and made to contract by artificial stimuli. When we have con- 

 sidered these, we can return to the lessons they teach us about the 

 normal contractions in our own bodies. 



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. 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 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 glycerine stimulates nerve, but 

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



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



