ELEMENTS OF PHYSIOLOGY 



have the same parts as the knee joint. It is rarely that two 

 bones put together so perfectly are forced from their 

 natural places. When this happens it is called a dislocation. 



116. The Kinds of Movable Joints are : the ball-and- 

 socket joint, which allows motion in any direction, as the 

 shoulder joint ; the hinge joint, which allows free motion 

 back and forth in one plane, as the knee joint ; the pivot 



joint, in which one bone rotates 

 around another, as the atlas 

 around the axis, the radius 

 around the ulna (Fig. 66); the 

 gliding joints, in which the flat 

 surface of one bone glides over 

 the other, as in the bones of the 

 wrist ; elastic joints (also called 

 mixed joints), in which there is 

 one flexible cartilage growing 

 to both bones, and motion is 

 allowed by the compression or 

 bending of the cartilage, as 

 where the ribs join the sternum. 

 Elastic joints are sometimes 

 called mixed joints, because 

 they resemble both movable 

 and immovable joints. There 

 are about 91 hinge joints, 4 ball- 

 and-socket joints, only 3 pivot 

 joints, and 43 elastic joints in 



the body. The gliding joints cannot be distinctly divided 



and numbered. 



117. Structure of a Long Bone. If a long bone is sawed 

 through lengthwise (Fig. 67), it will be observed that there 

 is a central cavity containing yellow marrow. (What kind 

 of steak has a marrow bone ?) We should observe also that 

 the portions inclosing this cavity are of a dense solid struc- 



FlG. 66. The Bones of the Right 

 Forearm. 



When the hand is supine A, and prone 

 B; H, humerus; R t radius; U, ulna. 



