102 IRRITABILITY AND CONTRACTILITY [CH. VIII. 



bility was enunciated by Haller more than a century ago, and was 

 afterwards keenly debated. It was finally settled by an experiment of 

 Claude Bernard which can be easily repeated by every student. 



If a frog is taken and its brain destroyed by pithing, it loses con- 

 sciousness, but the circulation goes on, and the tissues of its body 

 retain their vitality for a considerable time. If now a few drops of a 

 solution of curare, the Indian arrow poison, are injected with a small 

 syringe under the skin of its back, it loses in a few minutes all power of 

 movement. If next the sciatic or any other nerve going to muscle is 

 dissected out and stimulated, no movement occurs in the muscles to 

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

 that for all practical purposes the muscles are nerveless ; or rather 

 nervous impulses cannot get past the end-plates 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 Klihne. In part of some of the frog's muscles (e.g. part 

 of the sartorius) there are no nerves at all ; yet they 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- 



