IMMOVABLE JOINTS 247 



of attachment of the ligaments. In a ball and socket joint 

 the rounded head of one bone fits into a more or less con- 

 cave cavity in the other. Such a joint allows free movement 

 about this socket in every direction. The hip and shoulder 

 joints illustrate this type. In the hinge joint the bones 

 are so connected as to permit of motion in only two direc- 

 tions, just as a door swings upon a hinge. The elbow and 

 knee joints illustrate this type. In gliding joints the bones 

 slide over the surfaces of one another. The union of the 

 carpals in the WTist and of the tarsals in the ankle are ex- 

 amples in point. The lower jaw shows a combination of a 

 hinge and a gliding joint. The up and down movements 

 being a true hinge action while the power to protrude the 

 lower jaw beyond the upper is due to its gliding action. 



In the pivot joint one bone remains stationary and the 

 other rotates upon it. \Vhen we turn the wrist from right 

 to left this motion is made possible by such a pivotal joint 

 at the wrist at the point where the ulna and radius are in 

 contact. In this case, the radius rolls over the end of the 

 ulna carrying the hand with it while the ulna remains sta- 

 tionary. The movement of the atlas over the surface of the 

 fixed axis is another illustration of this t3rpe. (See p. 227) 

 Immovable joints. — These are of two kinds, sutures 

 and symphyses. The interlocking of the skull plates illus- 

 trates the former tj^pe. Here the plates are held in close 

 union by interlocking projections much as a board is dove- 

 tailed to fit another board. The projections of one bone 

 fitting into the spaces between projections of the other. 

 The symphysis type is illustrated in the union of the 

 ilia to the sacrum. Here the two smooth surfaces of the 

 bones are pressed tightly together and held in place by 

 muscles and ligament. 



