PHYSIOLOGY OF THE SKELETON. 4 < 



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 extremi- 

 ties. 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 joint 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 movement, varies in accordance with the form of the vertebrae and 

 the inclination of their articular processes. In the cervical and lum- 

 bar 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. 



The skeleton may, therefore be regarded as a highly developed 

 framework, which determines not only the form of the body, and 

 affords support and protection to the various softer organs and 

 tissues, but also, through the mobility of its joints, permits of a 

 great variety of complicated movements. 



