THE PHYSIOLOGY OF THE SKELETON. 47 



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 r61e. 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. Superi- 

 orly, 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 nbro-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 necessary 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 movement, varies in accordance with the 

 form of the vertebrae and the inclination of their articular processes. In the 

 cervical and lumbar regions extension and flexion are freely permitted, 

 though the former is greater in the cervical, the latter in the lumbar region, 

 especially between the fourth and fifth vertebrae. Lateral flexion takes place 

 in all portions of the column, but is particularly marked in the cervical 

 region. A rotatory movement of the column as a whole takes place through 

 an angle of about twenty-eight degrees. This is most evident in the lower 

 cervical and dorsal regions. 



