THE BONES. 27 



which he performs many of the duties of life. By this mem- 

 ber the blacksmith wields his heavy hammer, to fashion with 

 precision the piece of iron; and by it the musician, with 

 exquisite skill, produces upon his instrument the finest and 

 loftiest notes of melody. 



17. The Scapula and Clavicle. The arm is attached 

 to the body by a flat, triangular bone, the scapula or shoulder 

 blade, which rests upon the ribs on the back part of the 

 thorax, and is held to the side by a layer of muscles, thus 

 giving freedom of motion to the whole shoulder. The apex of 

 the scapula is marked by a pear-shaped concavity, which 

 forms with the upper end of the arm bone the shoulder joint. 

 This joint inclines somewhat forward, and is retained in posi- 

 tion by an ^/-shaped bone, the clavicle, or collar bone, which 

 extends from the shoulder blade inward to the breast bone, 

 and is easily seen on the uncovered neck. 



18. Number of Bones in the Upper Extremity. In 



addition to the bones which attach the upper extremity to 

 the body, the arm has thirty 1 Mines; one in the upper arm, 

 called the humerns ; two in the forearm, the ulna and radius, 

 lying side by side; eight in the wrist, the carpus ; five bones 

 form the hand, the metacarpus; and fourteen make up the 

 thumb and fingers, the phalanges. 



19. The Shoulder. At the shoulder we have a good 

 example of what is called a ball and socket joint, and the 

 degree of motion is here much greater than in any other part 

 of the body. Almost every inch of the surface of the body 

 can be reached by the fingers. To permit of such free motion, 

 the socket of this joint is quite shallow, and its articulating 

 surface limited, while the rounded head of the humerus has a 

 large articulating surface. Dislocations more frequently take 

 place in this joint than elsewhere, notwithstanding the pro- 

 vision existing to Keep i* **i place by ligaments and muscles. 



