CHAPTER X 



MUSCLE 



LIVING matter, protoplasm, is irritable. It responds to in- 

 fluences impressed upon it by its environment. An effective 

 influence, termed a " stimulus," produces a change in proto- 

 plasm at the spot at which it acts. From this spot the change 

 spreads outwards as an " impulse." Protoplasm is said to 

 " conduct." A stimulus may be likened to a blow given to a 

 fixed but elastic mass ; an impulse to the vibration which 

 travels outwards from the spot struck. Unfortunately, the 

 term "stimulus" is used both for the stick that strikes, the 

 stimulator, or stimulant, and for the blow that is struck ; but 

 breaches of logic seldom lead to confusion in an experimental 

 science. The context indicates the particular application of 

 the term. The manifestation of stimulation is a physical or 

 chemical change most obvious when it is one of form. This 

 change of form may occur at the spot stimulated, or may be 

 deferred to a distant part to which the impulse is conducted. 



In opening the study of muscle and nerve we need to form 

 a conception of the nature of these three functions irritability, 

 conductivity, and changeableness of form. Not that the 

 functions are as distinct as the ideas to which the three terms 

 give rise. They are three aspects of a common function ; 

 although this is a reflection which will carry more weight when 

 the ways in which protoplasm reacts to external forces have 

 been considered. 



A stimulus may be mechanical, something in the nature of 

 a blow which displaces the particles of protoplasm ; or it may 

 be chemical or thermal, disintegrating a portion of its sub- 

 stance ; or electrical, divorcing the ions of its molecules. Only 

 the last in any way resembles a natural stimulus ; since elec- 



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