These (Vertebræ coccygeæ) vary considerably in number, but eighteen may
be taken as an average. From first to last they become reduced in size
and, with the exception of a few at the beginning of the series, consist
of bodies only. The first three have bodies which are somewhat flattened
dorso-ventrally, constricted in the middle, and have at the ends
slightly convex, elliptical, articular surfaces. The ventral surface has
a median groove for the coccygeal artery. The arch is small and
triangular; it is formed of two flat plates which are prolonged to form
a short spinous process with a thickened and often double summit. The
anterior notches are absent. Functional articular processes are not
present, but small rudiments of the anterior pair commonly occur. The
transverse processes are relatively large plates which project
horizontally outward. Further back the arch becomes incomplete, open
above, and soon disappears; the transverse processes gradually fade out,
and the vertebræ are reduced to cylindrical rods of diminishing size.
The last one has a pointed end.
=Variations.=—The number is said by good observers to vary between
fourteen and twenty-one. In old age the first is often fused with the
sacrum, and sometimes with the second. The arch of the third may be
open.