THE PHYSIOLOGY OF THE SKELETON. 41 



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. 



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 circumduc- 

 tion; the latter joint permits of flexion and extension of the head, with 

 inclination to either side. When the surfaces do not take the same direc- 

 tion, 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 revolve 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. 



2. 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 united 

 and held in position largely by the intervention of a firm, elastic disc of 

 fibre-cartilage. Each joint is also strengthened by ligaments. 



3. Synarthroses. In this division are included all those joints in which 

 the 'opposing surfaces of the bones are immovably united, and hence 

 do not permit of any movement e. g., the joints between the bones 

 of the skull. 



