The =vertebral column= (Columna vertebralis) is the fundamental part of
the skeleton. It consists of a chain of median, unpaired, irregular
bones which extends from the skull to the end of the tail. In the adult
certain vertebræ have become fused to form a single bony mass with which
the pelvic girdle articulates. Vertebræ so fused are termed =fixed= or
=“false” vertebræ= (Vertebræ immobiles), as distinguished from the
=movable= or =“true” vertebræ= (Vertebræ mobiles).
The column is subdivided for description into =five regions=, which are
named according to the part of the body in which they are placed. Thus
the vertebræ are designated as =cervical=, =thoracic= (or dorsal),
=lumbar=, =sacral=, and =coccygeal= or =caudal= (Vertebræ cervicales,
thoracales, lumbales, sacrales, coccygeæ). The number of vertebræ in a
given species is fairly constant in each region except the last, so that
the =vertebral formula= may be expressed (for the horse, for example) as
follows:
C_{7}T_{18}L_{6}S_{5}Cy_{15–21}.
[Illustration: