MAN, A MAMMAL 



325 



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

 a lexer of the first 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 

 at t a<-hed 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 lever 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 t he 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- 

 tec lion 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. 



