The Skeletal System 105 



not provide sufficient support for a terrestrial animal. The terrestrial 

 animal, while needing additional support, also has the necessity of re- 

 taining the flexibility. Both flexibility and support have been provided 

 for by a backbone consisting not of a single bony element, but one 

 formed of many small individual parts or vertebrae. The cartilaginous 

 pads between the vertebrae act as shock absorbers, cushioning the jars 

 of movements. In man the curvatures of the spine lend additional 

 flexibility and also help to spread shocks throughout the entire verte- 

 bral column. 



As another function, the backbone gives complete protection to 

 the spinal cord by means of special arches which are dorsal to the main 

 body of the individual vertebrae. 



As another important function, the spinal column has innumer- 

 able surfaces for muscle attachment. These masses of muscle combined 

 with the bony elements form a strong, flexible body axis. 



In mammals, five different types of vertebrae are recognizable. These 

 are the cervical, thoracic, lumbar, sacral, and caudal. . The structure of 

 a thoracic vertebra may, in general, be studied as typical of that 

 of all the vertebrae. 



The main mass of the vertebra is the body or centrum. Dorsal to 

 this centrum is the neural canal surrounded by the curved neural arch. 

 Dorsally the neural arch terminates in the neural spine which provides 

 a surface for muscle attachment. From each side of the neural arch 

 extends a transverse process which also serves for muscle attachment. 

 Special smooth areas at the ends of each transverse process and on 

 each centrum serve for rib articulation. Each vertebra articulates with 

 its neighbor by means of two smooth surfaces : the superior and inferior 

 articular processes. The inferior articular process of one vertebra over- 

 laps the superior one of the following vertebra. Where the two verte- 

 brae articulate, an intervertebral foramen is present. This foramen is 

 a passageway for nerves entering and leaving the spinal cord. The 

 vertebrae of each region have this same general structure but are vari- 

 ously modified. 



With but few exceptions, all mammals have seven cervical ver- 

 tebrae. All have highly modified transverse processes which contain 

 an opening, the transverse foramen, through which the vertebral artery 

 passes. All except the first two have well-developed neural arches and 

 spines. The first two cervical vertebrae are highly modified for support 

 and movement of the skull. The first, the atlas, is ring-shaped and 

 has two smooth cuplike depressions which articulate with the occipital 



