The distinguishing mark of the _cervical vertebræ_ is the foramen in each transverse process, through which the vertebral arteries run to the skull. They are also smaller than the dorsal and lumbar vertebræ. The _dorsal vertebræ_ are distinguished by having on the transverse processes and on the body smooth articular surfaces called facets and demi-facets for articulation with the ribs. The _lumbar vertebræ_ are the largest and heaviest and have the thickest spine. By the time the sacral region is reached, however, the vertebræ have only a rudimentary spinous process. Moreover, in adult age the sacral bones grow together and form one triangular bone, the _sacrum_, which has a broad base called the _promontory of the sacrum_ and a blunt apex. It is concave in front and convex behind and has an articulating surface for joining the pelvic bones. In the case of the _coccyx_ also the four original bones, all rudimentary in character and supposed to be the survival of a tail, grow together to form one bone. Together the sacrum and coccyx form the posterior wall of the true pelvis. [Illustration: FIG. 32.--A type of vertebra. (Leidy.) 1, Body; 2, pedicle; 3, lamina; 4, spinal foramen; 5, spinous process; 6, transverse process; 7, articular process.] [Illustration: FIG. 33.--The sacrum, from before. (Drawn by D. Gunn.)] Some of the dorsal vertebræ are peculiar in the arrangement of their facets and demi-facets, while among the _cervical vertebræ_ are several whose peculiarities should be more carefully noted. Thus, the first cervical vertebra or _atlas_ supports the head and has practically no body, the place of the body being taken by a narrow anterior arch of bone and an opening, continuous with the spinal foramen, into which the _odontoid process of the axis_ fits, being held in place by ligaments. At either side on top is a facet for articulation with the occipital bone. There is almost no spine. The second vertebra or _axis_ has surmounting the body the odontoid process, with a facet in front for articulation with the atlas and one behind for the transverse ligament to move over. The seventh cervical vertebra or _vertebra prominens_ has a very long spinous process--hence name--to which is attached the _ligamentum nuchæ_. It can be felt very distinctly on the living. Running from the skull down through the spinal column into the sacral vertebræ and formed by the joining of the spinal foramina of the individual vertebræ is an opening called the _spinal canal_, which holds the cord. The cord, however, stops practically at the first lumbar vertebra, where it splits up into the _cauda equina_, only the _filum terminate_ extending farther down.