8 



Muscles 



I. introduction 



A very general property of all living matter is the ability it has to alter 

 its size or shape by contracting or expanding a given region of its body. 

 In most of the higher animals, certain cells or groups of cells are special- 

 ized to contract or relax, thereby changing the position and shape of the 

 animal. Other similar groups of cells contract and relax to pump 

 fluids (blood) through the animal, force food through the digestive tract, 

 and so forth. Aggregates of these specialized contractile cells are called 

 muscle tissues, or simply muscles. All other forms of protoplasm exhibit 

 a contractility similar to that of muscles, but the latter are specialized 

 to emphasize this property of contractility. Thus, contractility is 

 trivially obvious in human muscles but can also be demonstrated in all 

 living cells. 



Muscles have been of interest to biophysicists for many years; their study 

 will probably remain one of the fields of biophysical research for years 

 to come. Most of the earlier studies on muscles were part of a larger field 

 called biomechanics. This field was explored primarily by workers who, 

 because of their backgrounds and training, labeled themselves physiol- 

 ogists and anatomists. Today, biomechanics per se has passed out of 



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