THE PHYSIOLOGY OF THE SKELETON 49 



spiral 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 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. More- 

 over, 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 axis of rotation of the articulating surface 

 is parallel with, rather than transverse to the long axis of the bone, and 

 as the movement 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 movements 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. 



2. Biaxial Joints. In this group the articulating surfaces are unequally 



curved, though intersecting each other. When the surfaces lie in the 

 same direction, the joint is termed an ovoid joint e.g., the radio-carpal 

 and the atlanto-occipital. As the axes of these surfaces are vertical to 

 each other, the movements permitted by the former joint are flexion, 

 extension, adduction, and abduction, combined with a slight amount 

 of circumduction; the latter joint permits of flexion and extension of the 

 head, with inclination to either side. When the surfaces do not take the 

 same direction, the joint, from its resemblance to the surfaces of a 

 saddle, is termed a saddle-joint e.g., the trapezio-metacarpal. The 

 movements permitted by this joint are also flexion, extension, adduction, 

 abduction, and circumduction. 



3. Polyaxial Joints. In this group the convex articulating surface is a 



segment of a sphere, which is received by a socket formed by the 

 opposing articulating surface. In such a joint, termed an enarthrodial 

 or ball-and-socket joint e.g., the shoulder- joint, hip-joint the distal 

 bone revolves around an indefinite number of axes, all of which intersect 

 one another at the center of rotation. For simplicity, however, the 

 movement may be described as taking place around axes in the three 

 ordinal planes viz., a transverse, a sagittal, and a vertical axis. The 

 movements around the transverse axis are termed flexion and extension; 

 around the sagittal axis, adduction and abduction; around the vertical 

 axis, rotation. When the bone revolves around the surface of an 

 imaginary cone, the apex of which is the center of rotation and the base 

 the curve described by the hand, the movement is termed circumduction. 

 B. Amphiarthroses. In this division are included all those joints which 

 permit of but slight movement e.g., the intervertebral, the interpubic, 

 and the sacro-iliac joints. The surfaces of the opposing bones are 



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