84 IRRITABILITY AND CONTRACTILITY [CH. VIII. 



to muscle is dissected out and stimulated, no movement occurs in the 

 muscles to which it is distributed. Curare paralyses the end-plates, so 

 that nervous impulses cannot get past them and cause any effect on the 

 muscles. But if the muscles are stimulated themselves, they contract. 



Another proof that muscle possesses inherent irritability was 

 adduced by Kiihne. In part of some of the frog's muscles (e.g. part 

 of the sartorius) there are no nerves at all; yet these parts are 

 irritable and contract when stimulated. 



The evidence of the statement just made that the poisonous effect 

 of curare is on the end-plates is the following: The experiment 

 described proves it is not the muscles that are 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 circu- 

 lating blood all over the body except to the leg protected by the liga- 

 ture. 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 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 rmiscles 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. 



