PART II. HOW THE BODY IS MOVED 

 CHAPTER VI 



THE SKELETON 



94. Animals and their Skeletons. There are some ani- 

 mals, such as the slug (a kind of snail without a shell, 

 Fig. 50) and the jellyfish, that do not possess any hard part 

 corresponding to a skeleton. Such an animal, if it lives 

 on land, lies flat on the ground and moves slowly and with 

 difficulty. Some animals, as the crawfish and the beetle, 

 have the skeleton on the outside in the form of a crust. 



95. Three Functions of the Skeleton and the Three Shapes 

 of Bones. Even in man, so capable of self-protection, it 

 is necessary to have the skeleton, or hard part, to protect 

 some of the most delicate organs. The bones for this 

 purpose are flat, as the breastbone and shoulder blade, 

 the ribs, which protect the heart and lungs, and the skull 

 bones, which protect the soft and delicate brain. The 

 muscles by their shortening accomplish very simple and 

 imperfect motions ; by using the bones for support and as 

 levers, this motion is changed in rate, direction, and place 

 of application. 



The long bones of the arms and legs, with the fingers 

 and toes, have motion as their chief function. The ribs are 

 flat but are the longest bones in the body in proportion 

 to their size, and enable us to perform the important 

 motions of breathing. Yet they are classed as flat bones, 

 for they lack the round shaft and enlarged ends or heads, 

 which typical long bones have. Even the bones of the 



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