Caudal Vertebrae
There are approximately 20 caudal vertebrae in the dog. The cranial most individuals conform to a typical size and shape, but the caudal most vertebrae reduce gradually to simple rods and are atypical.
The body of the first caudal vertebrae is nearly as wide as it is long. A highly developed vertebral arch can be seen more readily in the first caudal segment. The lumen in which these arches enclose become progressively smaller until in the sixth or seventh caudal vertebra they represent only a small groove.
The cranial articular processes exist, although they have lost their articular function. Each vertebra also bears a mamillary process, which exists caudally in the series until all signs of the process have been diminished. There are also caudal articular processes which project from the caudal most border of the arch and are often asymmetric.
Hemal arches are present as separate bones that articulate with the ventral surfaces of the caudal ends of the bodies of the fourth, fifth, and sixth caudal vertebrae. These arches slope in such a way as to resemble a V or Y shape. These arches are important in protecting the median coccygeal artery, which passes through them. Found caudal to the hemal arches, and in corresponding position on succeeding vertebrae, are the paired hemal processes. These processes are the last features to disappear, and remnants of them can still be identified as far caudally as the seventeenth or sometimes the eighteenth caudal vertebra.