CHAPTER IV 



THE FRAMEWORK OF THE BODY, OR THE OSSEOUS SYSTEM 



31. Functions of the Bones. Certain parts of an animal 

 body are more essential to its existence than others, and 

 more important to its well-being. These portions are of 

 especial delicacy in substance and structure, and pecul- 

 iarly liable to injury. They therefore require protection. 

 For this purpose the more solid substances which make up 

 the body are arranged to inclose or shield the softer and 

 more delicate parts. 



In one large group of animal forms, called invertebrates, 

 which includes insects, mollusks, crabs, lobsters, etc., the 

 outer portion of the body consists usually of a more or 

 less hard and tough crust called the exoskeleton, which 

 covers the softer parts. But the higher group of animals, 

 called vertebrates or backboned animals, to which man 

 belongs, possess an inner, bony framework, called the en- 

 doskeleton, so arranged as to form a support and a defense 

 to the more sensitive and more essential parts. 



In all vertebrates the skeleton consists of a somewhat 

 firm but flexible bony column to which are attached the 

 bones of the head, the ribs, and the pectoral and pelvic 

 girdles, which connect respectively the upper and the 

 lower limbs with the trunk (Fig. 27). 



The bones of the skeleton furnish the necessary levers 

 and points of support for the muscles which are the organs 

 of motion. 



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