22 



SCIENCE PRIMERS. 



[§ HI. 



v;hich is stoutest at the wrist is called the radius. If 

 you look carefully you will see that the end of the 

 humerus at the elbow is curiously rounded, and the 

 end of the ulna at the elbow curiously scooped out, 

 in such a way that the one fits loosely into the 

 other. ' ■ ' 



F1G3.— /"-^^ Bottes of the Upper Extremity with the Biceps Muscle, 



The two tendons by which this muscle is attached to the scapula, or 

 shoulder-blade, are seen at a. P indicates the attachment of the muscle 

 to the radius, and hence the point of action of the power ; F, the ful- 

 crum, the lower end of the humerus on which the upper end of the 

 radius (together with the ulna) moves; W, the weight (of the hand). 



If you try to move them about one on the other, you 

 will find that you can easily double the ulna very 

 closely on the humerus without their ends coming 

 apart, and if you notice you will see that as you move 

 the ulna up and down, its end and the end of the 

 humerus slide over each other. But they will only 

 slide one way, what we may call up and down. IS 

 you try to slide them from side to side, you will find 

 that they get locked. They have only one movement, 



