xin BONES AND JOINTS 187 



of the seat must be so constructed that there is ample 

 room to sit well back on the seat, and at the same time 

 it must furnish a firm support for the loins. In this last 

 particular most of the school seats are especially faulty. 



SUMMARY OF THE MAIN POINTS 



1. The bones serve (i) to form the framework of the body, 

 (2) to protect certain soft and delicate organs, and (3) to form 

 levers for the attachment of muscles. 



2. The skeleton consists of two hundred and six bones fitted 

 together and held in place by cartilage and ligaments. 



3. Bones consist of animal and mineral matter. In early 

 childhood the animal matter is the more abundant, but it is 

 slowly displaced by mineral matter, which is far in excess in 

 old age. 



4. The long bones have a hollow shaft and enlarged, 

 spongy ends. They are covered with periosteum and lined 

 with endosteum, both of which are well supplied with blood 

 vessels from which the bone substance draws its nourishment. 



5. Cartilage is very much like bone that contains little min- 

 eral matter. It helps to enlarge the ends of long bones, and so 

 makes better joints, and to form the walls of some of the inter- 

 nal organs. 



6. A joint is the union of two bones, whether they move 

 upon each other or not. There are several different kinds of 

 joints, as ball-and-socket, pivot, hinge, saddle, gliding, and suture. 



7. A synovial sac occurs between the bones of some joints. 

 It contains a small quantity of synovial fluid which oils the 

 surfaces to prevent friction. 



8. The bones in childhood are very pliable and easily bent 

 out of position, and therefore it is highly important always to 

 maintain correct postures in lying, sitting, standing, and walking. 



