480 MANUAL OF PHYSIOLOGY. 



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 becomes 

 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 cor- 



Diagram of the Action 



ftheKneeJoint.-w= responds to the axis of motion. In ordinary 

 articular surface of fe- flexion the head of the tibia moves on the arc 

 P F = around the axis so as to partially relax the 



tibia in position of flex- lateral ligament and allow of some rotation 



km. c = centre of rota- on ^ ^ Qf ^ ^^ men ^ j^ Qf 



the tibia moves forward, in extension, it be- 

 comes 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- 

 ticular surface of the tibia proceeds forward. The effect of this 



