This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Normally the spinal column is composed of seven cervical, twelve dorsal, five lumbar, five sacral, and four to five coccygeal vertebrae. The sacral vertebrae tend to fuse together, forming a single bone, the sacrum. This fusion is complete at the twenty-fifth year. The coccygeal vertebrae join later, fusion occurring in middle life.
Sometimes in advanced age the coccyx and sacrum fuse together. The cervical vertebrae are almost always seven in number, but both the dorsal and lumbar vary much more frequently than is usually supposed. The occurrence of thirteen instead of twelve ribs on a side is not uncommon and I have seen skeletons with only eleven. A rudimentary cervical rib also occasionally occurs.
The tips of the spinous processes of the cervical vertebrae, the first two dorsal, and last four lumbar, pass almost horizontally backward and are therefore nearly opposite the bodies of the vertebrae to which they are attached. The tips of the spines from the third to the last dorsal inclusive, however, are opposite the bodies of the next vertebrae below them, being inclined downward, while the tip of the first lumbar is about opposite the intervertebral disk beneath.
At the third month of intra-uterine life there is only one large curve, convex posteriorly. At birth there are two curves, each convex posteriorly, a dorsal and sacral, probably to accommodate the thoracic and pelvic viscera; after the erect position is assumed the cervical and lumbar curves become established. The cervical passes into the dorsal curve at the middle of the second thoracic vertebra and the dorsal into the lumbar at the middle of the last thoracic vertebra. (Fig. 478). Laterally, there is a slight curve in the dorsal region with its convexity to the right, probably due to the increased use of the right hand.
Flexion and extension are free in the neck and lumbar region, rotation is slight in the cervical region, free in the upper portion of the dorsal, and gradually diminishes to be absent in the lumbar region.