CH. vni.] STIMULI. 109 



paralysed. It must therefore be either the nerves, or the links 

 between the nerve-fibres and the muscular fibres. By a process 

 of exclusion we arrive at the conclusion that it is these links, 

 for the following experiment shows it is not the nerves. The 

 frog is pithed as before, and then one of its legs is tightly 

 ligatured so as to include everything except the sciatic nerve of 

 that leg. Curare is injected and soon spreads by the circulating 

 blood all over the body except to the leg protected by the 

 ligature. It can get to the sciatic nerve of that leg because that 

 was not tied in with the rest. The sciatic nerve of the other leg 

 is now dissected out ; when the muscles supplied by it cease to 

 contract when the nerve is stimulated, the frog may be considered 

 to be fully under the influence of the drug. But on stimulating 

 the sciatic nerve of the protected limb, the muscles respond 

 normally ; this shows that the nerve which has been exposed to 

 the action of the poison has not been affected by it. 



Varieties of Stimuli. 



The normal stimulus that leads to muscular contraction is a 

 nervous impulse ; this is converted into a muscular impulse 

 (visible as a contraction) at 

 the end-plates. This nervous 

 impulse starts at the nerve- 

 centre, brain or spinal cord, 

 and travels down the nerve 

 to the muscle. In a reflex 

 action the nervous impulse 

 in the nerve-centre is started 

 by a sensory impulse from 



. . " Fig. 132. Muscle-nerve preparation, r, femur ; 



the periphery ; thus when *, nerve; T, tendo Achillis. (M'Kendrick.) 



one puts one's hand on some- 

 thing unpleasantly hot, the hand is removed ; the hot substance 

 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 considered 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 



