CHAPTER XV 

 THE MUSCULAR SYSTEM 



As already stated, the skeleton, the nervous system, and 

 the muscular system are concerned in the production of 

 motion. The skeleton and the nervous system, however, 

 serve other purposes in the body, while the muscular sys- 

 tem is devoted exclusively to the production of motion. 

 For this reason it is looked upon as the special motor sys- 

 tem. The muscular tissue is the most abundant of all the 

 tissues, forming about 41 per cent of the weight of the body. 



Properties of Muscles. The ability of muscular tissue 

 to produce motion depends primarily upon two proper- 

 ties the property of irritability and the property of 

 contractility. Irritability is that property of a substance 

 which enables it to respond to a stimulus, or to act when 

 acted upon. Contractility is the property which enables 

 the muscle when stimulated to draw up, thereby becoming 

 shorter and thicker (a condition called contraction), and 

 when the stimulation ceases, to return to its former condi- 

 tion (of relaxation). The property of contractility enables 

 the muscles to produce motion. Irritability is a condition 

 necessary to their control in the body. 



Kinds of Muscular Tissue. Three kinds of muscular 

 tissue are found in the body. These are known as the 

 striated, or striped, muscular tissue ; the non-striated, or 

 plain, muscular tissue ; and the muscular tissue of the 

 heart. These are made up of different kinds of muscle 

 cells and act in different ways to cause motion. The 



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