JOINTS 365 



Between the ends of the ribs and the sternum there are 

 cartilages of the shape of the ribs. In old age they often 

 take up lime and become real bone. 



653. Flexible joints. The joints of the head and trunk 

 of the body are mostly inflexible, while those in the limbs 

 permit very free movements of the bones. In flexible 

 joints the bones are held together by a strong fibrous 

 membrane called a ligament. The ends of the bone are 

 smooth and rounded so as to move freely upon each other. 



In some joints the movements are simply forward and 

 backward like a hinge. The fingers, toes, elbows, knees, 

 and ankles are hinge joints. 



In some joints the movements can be made forward and 

 backward and sideways like a ball in a socket. The 

 thumbs, great toes, shoulders, and hips have this kind of 

 a joint. In each the end of one bone is spherical and fits 

 into a hollow socket in the other. 



In other joints one bone can only rotate about another 

 as a pivot. In its union with the spine, the skull turns 

 about a fingerlike projection upon the top of the second 

 vertebra. At the elbow, the upper end of the radius 

 turns in a socket upon the side of the ulna through half 

 a circle of revolution. 



654. Structure of joints. In all flexible joints the liga- 

 ments pass from bone to bone, like a collar upon the out- 

 side of the bone, enveloping a cavity which is lined with 

 a thin and smooth membrane, called synovial membrane. 

 The synovial membrane secretes a fluid like the white of 

 an egg, called the synovial fluid. The fluid moistens and 

 lubricates the joint so that it turns smoothly and easily. 

 If it is absent the joint creaks when moved. 



655. Loose joints. The two bone surfaces of each joint fit 

 together accurately. There is a considerable difference in the depths 



