THE PHYSIOLOGY OF THE SKELETON. 61 



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. 



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 fibro- 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 

 the middle of the back of the trunk, it forms the foundation of the 

 entire skeleton. It is composed of a series of superimposed bones, 

 termed vertebrae, which increase in size from above downward as 

 far as the brim of the pelvic cavity. Superiorly, it supports the skull; 

 laterally, it affords attachment for the ribs, which in turn support the 

 weight of the upper extremities; below, it rests upon the pelvic bones, 

 which transmit the weight of the body to the inferior extremities. 

 The bodies of the vertebrae are united one to another by tough elastic 

 discs of fibro-cartilage, which, collectively, constitute about one- 

 quarter of the length of the vertebral column. The vertebrae are held 

 together by ligaments situated on the anterior and posterior surfaces 

 of their bodies, and by short, elastic ligaments between the neural 

 arches and processes. These structures combine to render the 

 vertebral column elastic and flexible, and enable it to resist and 

 diminish the force of shocks communicated to it. 



The amphiarthrodial character of the intervertebral joints endows 

 the entire column with certain forms of movement which are neces- 

 sary to the performance of many body activities. While the range 

 of movement between any two vertebrae is slight, the sum total of 

 movement of the entire series of vertebrae is considerable. In different 

 regions of the column the character, as well as the range of move- 

 ment, varies in accordance with the form of the vertebrae and the 

 inclination of their articular processes. In the cervical and lumbar 



