108 IRRITABILITY AND CONTRACTILITY. [CH. vin. 



slowly ; each drop has to get bigger before it possesses enough 

 energy to fall. Thus we may get different degrees or rates of 

 rhythmic movement. So in the body, during the period of rest, 

 the cilium or the heart is accumulating potential energy, till, as it 

 were, it becomes so charged that it discharges ; potential energy 

 is converted into kinetic energy or movement. 



When contraction travels as a wave along muscular fibres, or 

 from one muscular fibre to another, the term peristalsis is 

 employed. These waves are well seen in such a muscular tube as 

 the intestine, and are instrumental in hurrying its contents along. 

 The heart's contraction is a similar but more complicated 

 peristalsis occurring in a rhythmic manner. 



The physiology of muscle and nerve furnish us with the best 

 means of studying irritability and contractility. We shall have 

 to consider these two tissues together to a large extent, but must 

 confine our attention at the outset to the voluntary muscles. 



The question may be first asked, what evidence there is of 

 irritability in muscle ? May not the irritability be a property of 

 the nerve-fibres which are distributed throughout the muscle and 

 terminate in its fibres ? The doctrine of independent muscular 

 irritability 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 

 consciousness 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 Kiihne. 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 



