GENERAL STRUCTURE OF THE BODY. 19 



ing various salts of calcium, magnesium, and sodium. 

 In young children the animal matter predominates and 

 the bones are soft and often bend instead of breaking, 

 only the outside shell on one side giving way, as in " green- 

 stick" fracture. In rickets there is a deficiency of lime 

 salts, but the increased brittleness of the bones in old 

 age is due, not to increase of mineral matter, but to the 

 less spongy texture of old bones. 



Classification and Function of Bones. There are in the 

 body some two hundred bones, which may be classified 

 as long, short, flat, and irregular. Occasionally an ir- 

 regular bone develops in a fontanelle, the membranous 

 opening at the juncture of the sutures of the skull. 

 This is known as a Wormian bone. It is not, however, 

 included in the two hundred, as are not the sesamoid 

 bones or bones developed in tendons, with the exception 

 of the patella or knee-cap. 



Long bones are developed in cartilage and consist of a 

 shaft, two extremities, and various processes. They are 

 more or less curved to give them strength and grace. 

 They serve as supports and act as levers for purposes of 

 motion and the exercise of power. Since a hollow cyl- 

 inder is just as strong as a solid one of the same size, the 

 weight coming only on the outer shell, the great bones 

 which are accountable for weight and which need to be 

 light themselves have hollow shafts, composed chiefly of 

 compact tissue with a central medullary canal. The 

 ends, however, are expanded in order to make better 

 connection at the joints and to afford broad surfaces 

 for muscular attachment-, cancellous tissue being used 

 in them for lightness and strength. The large spongy 

 ends also give elasticity and lessen jar, and by bringing 

 the tendons to the bone at a greater angle increase their 

 effectiveness. Blood is brought to the long bones not 

 only by the vessels of the periosteum but by the medul- 

 lary artery, which penetrates the compact tissue by the 

 nutrient foramen and divides into an ascending and a 

 descending branch. 



