CHAPTER XLV 



SKELETAL SYSTEM. 



General. You are all familiar with animals which 

 have no hard matter in their bodies at all, such as the 

 jelly-fishes and worms. Other animals such as insects, 

 crayfishes, and clams, have a hard outer covering. This 

 outside skeleton is preferable to none because it gives 

 the animal some rigidity and some protection. It is 

 very inconvenient, however, for in most cases it does 

 not permit of any growth. As a result it must fre- 

 quently be shed, and then for a short period the animal 

 is left with only a soft outer covering. 



In the higher animals there is a well developed inner 

 framework of bones. In the human body this skeleton 

 is composed of over two hundred separate bones. These 

 are joined and serve several purposes: 1. They give 

 shape and rigidity to the body. 2. They protect the 

 delicate organs of the body. 3. They provide places for 

 the attachment of muscles, and serve as levers upon 

 which the muscles may act. 



Regions of the skeleton. For convenience the bones 

 of the skeleton may be divided into three groups, 

 namely : 



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