THE BONY FRAMEWORK 



35 



The joint end of the bones is smooth, moist, and tipped 

 with a thin layer of cartilage, called hyaline cartilage. 

 This smooth and glistening covering is bathed with a 

 sticky fluid called the synovial fluid, so named because it is 

 like the white of a raw egg. This is the liquid often 

 spoken of as "joint oil," furnished by 

 nature to allow the rubbing surfaces 

 to move smoothly over one another, 

 and thus prevent too much wear and 

 tear. 



57. Different Kinds of Joints. There 

 are two principal kinds of joints, the 

 imperfect, or practically immovable, and 

 the perfect, or movable. Thus, the bones 

 of the head, as we have seen, are firmly 

 dovetailed by jagged edges, which 

 grow into each other from infancy. 

 These are known as imperfect joints. 



Movable joints allow the bones to 

 glide on each other with more or less 

 freedom of motion, 

 cording to the motion needed. 



Thus, the joint at the hip is called 

 a ball-and-socket joint, and is not unlike 

 a child's toy cup and ball, because 

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

 the hip bone. The rounded head of the arm bone, as we 

 have learned, works in the shallow, saucerlike cavity of 

 the shoulder blade. Such a joint allows a great variety 

 of motion in almost every direction. 



Bones at certain joints are grooved and ridged so that 

 one bone can glide over the other to and fro, like the lid of 



They differ ac- Fia 27 ' Showin s how 



the Ends of the Bones 

 are shaped to form 

 the Elbow Joint. 



The cut ends of a few 

 ligaments are seen. 



