4O HUMAN PHYSIOLOGY. 



Classification of Joints. All joints may be divided, according to the 

 extent and kind of movements permitted by them, into (i) diarthroses; 

 (2) amphiarthroses; (3) synarthroses. 



i. Diarthroses. In this division of the joints are included all those which 

 permit of free movement. In the majority of instances the articulating 

 surfaces are mutually adapted to each other. If the articulating surface 

 of one bone is convex, the opposing but corresponding surface is concave. 

 Each surface, therefore, represents a section of a sphere or cylinder, 

 which latter arises by rotation of a line around an axis in space. Accord- 

 ing to the number of 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 extension 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 coin- 

 cident, 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 spirial hinge, or helicoid. As 

 the articulating surfaces of the condyles 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 move- 

 ment 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 move- 

 ment 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 



