SKELETAL MOVEMENTS. 



477 



other bone, while the sides of the joint are kept tightly together 

 by means of strong lateral ligaments. Examples of this form of 

 joint are to be found between the phalanges of the digits and at 

 the humero-ulnar joint. 



3. The rotatory hinge, or pivot joint, is that in which a part 

 moves round the axis of a bone, instead of the axis of rotation 

 being at right angles to both bones, forming the joint as in an 

 ordinary hinge. Such joints are seen at the head of the radius 

 and at the articulation between the atlas and the odontoid pro- 

 cess of the axis. 



4. A saddle-shaped joint is a kind of double-hinge, in which 

 each of the two articulating bones form a partial socket and 

 roller, and hence there are two axes of rotation placed more or 

 less at right angles one to the other. A good example of this 

 kind of joint occurs between the thumb and one of the wrist 

 bones. 



5. Spiral articulations are modifications of the hinge, in which 

 the surface of the roller does not run "true," but becomes eccen- 

 tric, so that the surface of the roller forms 



really a part of a spiral by means of which 

 the bone articulating with it is forced away 

 from the central axis of rotation and be- 

 comes jammed as if stopped by a wedge. 

 The best example of this is the knee. In 

 this joint the axis of rotation is near the 

 posterior surfaces of the bones, and passes 

 transversely through the condyles of the 

 femur, the surfaces of which form an arc, 

 the centre for which corresponds to the axis 

 of motion. In ordinary flexion the head of 

 the tibia moves on the arc around the axis 

 so as to partially relax the lateral ligament 

 and allow of some rotation on the axis of 

 the tibia. When the head of the tibia moves 

 forwards, in extension, it becomes wedged 

 against the anterior part of the articular 

 surface of the femur, which presents an eccentric spiral-like curve, 

 departing more and more from the centre of rotation as the ar- 



FIG. 195. 



Diagram of the 

 action of the knee- 

 joint. w= articular 

 surface of femur. E 

 = tibia in position of 

 extension. F = tibia 

 in position of flexion, 

 c = centre of rota- 

 tion. 



