THE SKELETAL SYSTEM 37 



Individual vertebra in each group closely resemble each other. 

 All the vertebrae are constructed on a common plan, so a description 

 of a typical member will serve as a basis, and any marked devia- 

 tion from this plan will be noted afterward. 



A typical vertebra presents the following features; A body, which 

 is a cylindrical mass of bone upon which the other parts are built 

 up. The upper surface of the body is flattened and forms the floor 

 of the spinal canal. The anterior extremity presents a convex 

 head to articulate with the posterior concave surface of the pre- 

 ceding vertebra. Between each two vertebrae there is a cartilagi- 

 nous disk which binds the bones firmly together. An arch, which 

 is built up on the upper surface of the body and completes the 

 canal for the spinal cord. The processes, which are of three varieties : 

 (1) articular processes, which project forward and backward from 

 the arch; (2) transverse processes, usually two in number, which 

 project outward from the lower part of the arch; (3) a spinous 

 process, which is single and projects upward from the arch. 



The cervical vertebra form the skeleton of the neck. There are 

 seven bones in this group. The first two are modified so as to allow 

 the skull to move more freely. A typical member of this group 

 has an extremely long body with a large, strong arch and spinal 

 canal. The articular processes are well developed. The trans- 

 verse processes are wide and plate-like. Each spinous process is 

 small. The first cervical vertebra is called the atlas and has no 

 body, instead it is modified in the form of a ring. On each side 

 of the anterior edge of the ring is a deep articular cavity for a 

 condyle of the occipital bone of the skull. The posterior edge ar- 

 ticulates with the axis. The axis, or second cervical vertebra, has 

 a very long body. The anterior extremity presents centrally a 

 projection called the odontoid process. The posterior surface 

 has the usual concave articular surface. The spine is prominent, 

 wide, and thick. 



The thoracic or dorsal vertebrae are usually eighteen in number, 

 sometimes nineteen, rarely seventeen. The body is short and pre- 

 sents laterally two concave facets for articulation with the heads 

 of the ribs. The arch is small. The articular processes are slightly 

 developed; the transverse processes are short and thick, and each 

 has a facet for articulation with the tubercle of a rib; the spinous 

 process is long, and together they determine the contour of the 

 back and withers. 



