MAN, A MAMMAL 



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the seesaw is an example of this. The best example in the human body of 

 a le\er of the fli-st class is seen when the head nods. Here the fulcrum is 

 the vertebra known as the atlas; the power is the muscles of the neck 

 attached to the back of the skull and to the spine ; the weight is the front 

 part of the head. When one keeps the head erect, this lever is used ; 

 the nodding head when one is napping shows this plainly. 



\ \ 



B C 



Three classes of levers. A, a lever of the first class; B, a lever of the second class; 

 C, a lever of the third class. (See text.) 



A lever of the second class has the fulcrum at one end, and the weight 

 between it and the power ; when we rise on our toes, we use this kind of 

 lever. 



In a le%'er of the third class, the fulcrum is at one end, with the power 

 between it and the weight. This is the kind of lever seen most frequently 

 in the human body. The flexing (drawing up) of the lower leg or the fore- 

 arm is an example of the use of this kind of lever. In such a lever, a wide 

 range of movement is obtained. 



General Structure and Uses of the Skeleton. — Evidently bones 

 form a framework to which muscles are attached; thus they are 

 used as levers for pur- 

 poses of movement. 

 Second, they give pro- 

 tection to delicate or- 

 gans ; they form a case 

 around the brain and 

 spinal cord; as ribs 

 they protect the or- 

 gans in the body 

 cavity. Third, they 

 give rigidity and form 



to the body. The skeleton of a dog; a. typical mammal. 



