26 SCIENCE PRIMERS. [§ in. 



trae that the end of the humerus moves over the end of 

 the radius as well as over the end of the ulna, and that 

 the end of the radius is also coated with cartilage and 

 is included in the wrapping of the ligaments. I 

 might add that the radius also moves independently 

 on the ulna, but I don't want to trouble you with this 

 just now. What I wanted to show you was that the 

 elbow is a joint, a joint so constructed that it allows 

 the fore-arm to be bent on the upper arm. "-■ 



i6. In order that the arm may be bent, 

 some force must be used. The ulna or radius — 

 for the two move together — must be pushed or pulled 

 towards the humerus, or the humerus must be pushed 

 or pulled towards the radius and ulna. How is this 

 done in your own arm ? 



Take the bones of the arm ; fix the top end of the 

 humerus ; tie it to something so that it cannot move. 

 Fasten a piece of string to either the radius or ulna 

 (it doesn't mutter which), rather near the elbow. 

 Bore a hole through the top of the humerus and pass 

 the string through it. Your ^ring must be long 

 enough to let the arm be quite straight without any 

 strain on the string. Now, taking hold of the 

 string where it comes out through the humerus, 

 pull it. The fore-arm will be bent on the arm. 

 Why? Because you have been working a 

 lever of the third order. 



The radius and ulna form the lever ; its fulcrum is 

 the end of the humerus in the elbow (Fig, 3, F) ; the 

 weight to be moved is the weight of the radius and 

 ulna (with that of the bones of the hand if present), and 

 this may be represented by a weight applied at about 

 the middle of the fore-arm ; the power is the pull 



