THE SKELETON 265 



As will be pointed out later (page 275), the muscles which 

 produce bending are not necessarily located near the joint 

 they operate; such an arrangement would often result in a 

 difficult and bungling sort of movement. 



Since accidents at movable joints are always likely to be 

 serious because they are apt to cause stiffness, we shall ex- 

 amine one or two carefully in order to learn their mechanism. 

 Perfect joints are of several different kinds, but most of them 

 are modifications of three simple types: the hinge joint, the 

 ball-and-socket joint and the pivot joint. 



The Hinge Joint. In the hinge joint the bones are able to 

 move back and forth in one direction only, like a door on 

 hinges: the knee, the elbow and the joints of the fingers are 

 good examples. Since all hinge joints are very much alike 

 in structure, the description of one will show the salient 

 features of all. 



The knee joint is made up of the femur and the tibia bones; 

 Fig. 131. The lower end of the femur is large and rounded 

 in one direction at the end, while the upper end of the tibia is 

 slightly hollowed on top; when the bones are placed together, 

 their shape permits movement only in one direction. The 

 rounded ends of both bones are covered with a thin layer of 

 cartilage, making movement easier. Two separate ring-like 

 cartilages, the semi-lunar fibro-cartilages, one on the outside 

 and one on the inside of the leg, furnish extra padding to 

 relieve the body of jars, and also fill up the spaces, making 

 the joint more compact. In the living joint, there is wrapped 

 around the ends of the bones the synovial membrane, which 

 secretes into the joint a liquid, the synovial fluid, the purpose of 

 which is to moisten the surfaces and prevent friction. The 

 free motion of the joint is dependent upon the presence of 

 this fluid and if for any reason the membrane ceases to secrete 

 it, friction develops, motion becomes difficult, inflammation 

 sets in, and eventually the bones are likely to grow together 

 and the joint to become stiff. All of the parts are evidently 



