CHAPTER II. 

 VOLUNTARY MUSCLE. 



One of the most striking characteristics of living things is 

 their power of movement. This is not so marked in plants, but 

 all animals, even those which lack the ability to move from place 

 to place, possess organs which are constantly in motion. The power 

 of contractility which is inherent in all protoplasm is developed 

 to its highest degree in muscular tissue. Thus practically every 

 life process which involves the movement of masses of tissues is 

 brought about by the action of muscles, e.g., breathing, circulation 

 of the blood, passage of the food down the alimentary tract, etc. 



A portion of the muscle of the body is under the control of 

 the will. Upon this muscle we depend for locomotion, movement 

 of our hands, eyes and the like. Such muscle is distinguished by 

 the fact that it is composed of fibres with a characteristic cross 

 striation. 



STIMULATION. 



Various kinds of stimuli will cause muscle to contract, but before 

 deciding upon the best form to use we must know where to apply 

 the stimulus. Normally the muscle fibres are called into action 

 by impulses passing along the nerve fibres. Artificial stimuli can 

 be effectively applied along the same path. Thus mechanical, 

 thermal, chemical and electrical changes can cause a muscle to 

 act through its nerve. Any of these changes to be effective must 

 not only be of sufficient magnitude but must be abrupt. A nerve 

 slowly compressed may be without effect, but if quickly pinched, 

 a twitch of the muscle results. 



Of all artificial stimuli, an electric current is not only the most 

 effective, but also the least injurious to nerve. Mechanical and 

 thermal changes great enough to have an effect injure the tissue 

 permanently so that these stimuli can be used only a single time. 

 Chemical stimulation if not harmful, is at least difficult to control. 

 Moderate electrical changes have none of these objections. 



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