46 HUMAN PHYSIOLOGY. 



2. Biaxial Joints. In this group the articulating surfaces are un- 

 equally curved, though intersecting each other. When the sur- 

 faces 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 abduc- 

 tion, 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 move- 

 ments 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 rota- 

 tion. For simplicity, however, the movement may be described as 

 taking place around axes in the three ordinal planes viz., a trans- 

 verse, 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. 



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 oppos- 

 ing bones are united and held in position largely by the intervention 

 of a firm, elastic disc of nbro-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. 



The Vertebral Column. In all static and dynamic states of the 

 body the vertebral column plays a most essential role. Situated in 



