In the foal the bodies of the five sacral vertebræ form joints which
resemble somewhat those in the posterior part of the lumbar region.
These joints are invaded by the process of ossification early, so that
the consolidation of the sacrum is usually complete, or nearly so, at
three years.
The coccygeal vertebræ are united by relatively thick intervertebral
fibro-cartilages, which have the form of biconcave discs. Special
ligaments are not present, but there is a continuous sheath of fibrous
tissue. The movement in this region is extensive and varied. In old
horses the first coccygeal vertebra is often fused with the sacrum.