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20 are separated by a certain amount of fibrous tissue forming the intervertebral disc, and the apposed surfaces of the centra are as a rule nearly flat. In this last feature, and in the important fact that the centra are ossified from three distinct centres, the anterior and posterior pieces ("epiphyses") remaining distinct for a time, even for a long time (as in the Whales), the centra in the mammals differ from those of reptiles and birds. The epiphyses are not found throughout the vertebral column of the lowly-organised Monotremata, and they do not appear to exist in the Sirenia.

From each side of the centrum on the dorsal side arises a process of bone which meets its fellow in the middle line above, and is from there often prolonged into a spine. A canal is thus formed which lodges the spinal cord. This arch of bone is known as the neural arch, and the dorsal process of the same as the spinous process. The sides of the neural arch bear oval facets, by which successive vertebrae articulate with one another: those situated anteriorly are the anterior zygapophyses, while those on the posterior aspect of the arch are the posterior zygapophyses; these articular facets do not exist in the tail-region of many mammals, e.g. Whales.

In addition to the dorsal median spinous process of the