PHYSIOLOGY OF THE SKELETON. 45 



axes around which the movements take place all diarthrodial joints 

 may be divided into : 



i. Uniaxial Joints. In this group the convex articulating surface is 

 a segment of a cylinder or cone, to which the opposing surface more 

 or less completely corresponds. In such a joint the single axis 

 of rotation, though, practically is not exactly at right angles to the 

 long axis of the bone, and hence the movements flexion and ex- 

 tension which take place are not confined to one plane. Joints 

 of this character e. g.,the elbow, knee, ankle, the phalangeal joints of 

 the fingers and toes are, therefore, termed ginglymi, or hinge- 

 joints. Owing to the obliquity of their articulating surfaces, the 

 elbow and ankle are cochleoid or screw -ginglymi. Inasmuch as the 

 axes of these joints on the opposite sides of the body are not 

 coincident, the right elbow and left ankle are right-handed screws ; 

 the left elbow and right ankle, left-handed screws. In the knee- 

 joint the form and arrangement of the articulating surfaces are 

 such as to produce that modification of a simple hinge known as a 

 spiral hinge, or helicoid. As the articulating surfaces of the con- 

 dyles of the femur increase in convexity from before backward, 

 and as the inner condyle is longer than the outer, and therefore, 

 represents a spiral surface, the line of translation or the movement 

 of the leg is also a spiral movement. During flexion of the leg 

 there is a simultaneous inward rotation around a vertical axis 

 passing through the outer condyle of the femur ; during extension 

 a reverse movement takes place. Moreover, the slightly concave 

 articulating surfaces of the tibia do not revolve around a single 

 fixed transverse axis, as in the elbow-joint, for during flexion they 

 slide backward, during extension forward, around a shifting axis, 

 which varies in position with the point of contact. 



In some few instances the long axis of the articulating surface 

 is parallel rather than transverse to the long axis, and as the 

 movement then takes place around a more or less conic surface, 

 the joint is termed a trochoid or pulley e. g., the odonto-atlantal 

 and the radio-ulnar. In the former the collar formed by the atlas 

 and its transverse ligament rotates around the vertical odontoid 

 process of the axis. In the latter the head of the radius revolves 

 around its own long axis upon the ulna, giving rise to the move- 

 ments of pronation and supination of the hand. The axis around 

 . which these two movements take place is continued through the 

 head of the radius to the styloid process of the ulna. 



