The Coccyx, usually composed of four fused segments, is a triangular bone which articulates with the Sacrum above and is free at its distal extremity. Its portion of the neural canal is open posteriorly and contains merely the thread-like termination of the cord membranes. It is frequently ankylosed to the Sacrum, sometimes in an abnormal position so as to impinge the single pair of coccygeal nerves.
The different regions of the spine show decided differences in structure, though all resemble each other. The Cervicals are smallest, the Dorsals next in size, and the Lumbars largest and strongest of the movable vertebrae. The Dorsals have facets and demi-facets for the articulation of the twelve pairs of ribs with their bodies and intervertebral substance, as well as oval facets upon the anterior aspect of their transverses for articulation with the tubercles of the ribs.
The spinous processes are smallest and usually bifurcated down to and including the fifth. The sixth may show a plain bifurcation, or on any Cervical the bifurcation may be so small as to be imperceptible to touch. The spinous process of the second overlies that of the third so as to make the latter very difficult of detection. Indeed, all cervical spinous processes down to the sixth are harder to palpate than those in other regions, owing to the anterior cervical curve. The processes lie in a groove between prominent muscle ridges.
Dorsal spinous processes are usually single, although the last four, three, two, or one may show plain bifurcation in certain individuals. They are somewhat pointed and overlap, except the lower ones, the obliquity being greatest in the mid-dorsal region and least at the first and last dorsals.
Lumbar vertebrae have broad, flat-tipped spinous processes much larger than the others. The last Dorsal may sometimes appear like a Lumbar in shape, so that the change in shape commonly supposed to mark a division between Dorsals and Lumbars is not always an infallible guide.
The transverse processes in the cervical region are very short and lie close in front of the articular processes. They are pierced by foramina for the vertebral artery and vein, except the seventh, which may have one foramen or none. They are difficult of access for palpation because of their shortness and the amount of overlying muscle, but may be reached from the front and side by drawing back the sternomastoid. They increase in length from the second to the seventh.
In the dorsal region the transverses are larger and stronger and more constant in size, shape, and direction, serving to support rib articulations. They extend in a curved direction outward, backward, and slightly upward from the union of laminae and pedicles and terminate in a large subcutaneous club-shaped extremity which may be readily palpated. The eleventh and twelfth dorsal transverses do not articulate with the ribs and must therefore be used with caution or not at all as levers for adjustment. The dorsal transverses are located on a higher level than the spinous processes. In the case of the upper three dorsals the transverse lies in a plane which would cross the mid-spinal line between its own and the next superior spinous. In the mid-dorsal region the transverse is even with the spinous of the vertebra above, though the relation may vary slightly. The lower dorsals return to the same relation as the upper.
The transverse processes of the Lumbars are relatively light compared with the general structure of the vertebrae and are found just even with the interspace between their own and the adjacent superior spinous process. They vary greatly in size, length and strength and may be used as levers for adjustment only when they are large enough to be clearly palpable through the muscle mass which separates them from the body surface.