THE FRAMEWORK OF THE BODY 241 



of all the muscle fibers that causes the muscle to shorten. The 

 tendons themselves are not elastic. There are nearly 300 

 skeletal muscles in the body and they occur almost invariably 

 in pairs, thus acting as antagonists to each other. One pulls 

 a bone in one direction and the other returns it to its original 

 position. Most muscles end in tendons, but all do not do so. 

 The diaphragm, one of the most important muscles in the body, 

 is such an exception. It is located just below the heart and 

 lungs and separates the body cavity into two chambers called 

 respectively the abdominal and thoracic cavities. Certain 

 other muscles are designed to regulate the size of various 

 tubes and openings and are known as sphincters. Such a 

 muscle is found where the stomach opens into the intestine. 



205. Value of Exercise. In order to contract, a muscle 

 must receive a stimulus from the brain. When a stimulus 

 thus causes a muscle to contract, it remains in that condition 

 for about }{Q of a second. A longer period of contraction 

 requires additional stimuli. When a muscle is used continu- 

 ously for a time it becomes fatigued and rest is necessary. A 

 change of work which puts other muscles into action may serve 

 the same purpose as a complete rest. Regular exercise is 

 beneficial to the muscles since it educates the cells to work in 

 harmony and promotes their development. Muscles which 

 are used regularly are larger, firmer, and darker colored 

 than those which are not so used. In developing the muscles, 

 a little exercise each day is much better than more vigorous 

 exercise at irregular intervals. Rowing, swimming, walking, 

 and running are desirable forms of exercise because they call 

 so many muscles into action. Many forms of exercise are 

 taken for the pleasure they give, such as walking, dancing, 

 skating, and many of the games played by children. 



206. Breaks, Sprains and Deformities. When a bone is 

 broken, it must be held immovably in one position until the 

 bone cells can form new matter with which to knit the broken 



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