42 Life and Health 



60. Synovial Membrane. A very delicate layer of cells 

 supported by connective tissue, called the synovial membrane, 

 lines the capsules of the joints and covers the ligaments 

 connected with them. It secretes the synovia, or joint oil, 

 a thick and glairy fluid, something like the white of a raw 

 egg in consistency, which moistens the inner surfaces of 

 the joints. Every part of a joint is thus enabled to run 

 with very little friction. 



61. Ligaments. The bones are held in place, and their 

 movements limited to a certain extent, by bands of various 

 forms, called ligaments. These are composed mainly of 

 bundles of white fibrous tissue placed parallel to, or closely 

 interlaced with, one another, and present a shining, silvery 

 aspect. 



While ligaments are pliable and flexible, permitting free 

 movement, they are also wonderfully strong, tough, and 

 inextensible. A bone may be broken, or its end torn off, 

 before its ligaments are ruptured. 



62. Different Kinds of Joints. It is only perfect joints 

 that are fully equipped with the structures just men- 

 tioned. Some joints lack one or more, and are there- 

 fore called imperfect joints. Such joints allow little or no 

 motion. 



The so-called perfect joints vary according to the nature 

 and amount of movement permitted. They are divided 

 into hinge joints, ball-and-socket joints, and pivot joints. 



The hinge joints allow forward and backward movements 

 like a hinge. These joints are the most numerous in the 

 body, as at the elbow, the ankle, and the knee. 



In the ball-and-socket joints a beautiful contrivance 

 the rounded head of one bone fits into a socket in the 

 other, as the hip joint and the shoulder joint. These 

 joints permit free motion in almost every direction. 



