IRRITABILITY 3!? 



The Irritability of Muscle 



Striated muscle retains, to a considerable degree, the 

 primitive characteristic of protoplasm in general — irrita- 

 bihty — though this is normally masked by the superior 

 irritabihty of nerve. The inherent irritabiUty of muscle 

 is shown by the occurrence of contraction in strips of 

 muscle demonstrably free from nervous element. It is 

 shown most perfectly by Claude Bernard's classical experi- 

 ment. In a frog both sciatics were exposed and a ligature 

 tied round the right thigh so as to include all tissues except 

 the nerve. Curare was then injected into the lymph sacs. 

 In a few minutes stimulation of the left sciatic nerve was 

 without effect upon the gastrocnemius, while on the right 

 side a normal contraction was evoked. On both sides 

 the muscles continued to respond to direct stimulation. 

 The drug had therefore paralysed neither the muscles nor 

 the nerve-trunk, since the muscles had been exposed to its 

 action on the left side and the nerve-trunk on both sides. 

 It had acted upon the nerve-endings in the muscles. 

 These having been put out of action, direct stimulation 

 affected the muscle itself. 



CHANGES ACCOMPANYING CONTRACTION 



The changes which a muscle undergoes when it passes 

 from the uncontracted to the contracted state may be thus 

 enumerated— 



1. Change of form. 



2. Development of tension. 



3. Change in excitabihty. 



4. Chemical changes. 



5. Electrical changes. 



6. Thermal change. 



We now have to consider each of these in turn, pointing 

 out how the information gained leads us to an understanding 

 of the nature of contraction. 



