Notes on Anatomy and Physiology: The Vertebrae

The last post examined the spinal column and the central role it plays in everyday life. We will now turn to the small bones that make up this column – the vertebrae.

The 24 vertebrae that connect skull to sacrum vary greatly in shape and function. They do, however, share 2 major features: a vertebral body in front and a vertebral arch behind.

Fig 1 Views of a typical vertebra, the 6th thoracic, from above (on the left) and from the side (on the right). You see the vertebral body out in front; the vertebral arch and its attachments lie behind. Note that the superior facets of one vertebra, facing backwards, are designed to overlap with the forward facing inferior facets of the vertebra above. Like shingles on a roof. Netter, 2006, Plate 154

Each vertebral body is a box-shaped block of bone with essentially flat surfaces top and bottom. An outer shell of solid, cortical bone surrounds an internal space occupied by a web of trabecular bone – thin bony struts that run both vertically and across. This design provides a lightweight structure that involves a minimum of material (bone) in its construction. And yet the structure is strong and able to resist collapse when asked to bear the body’s weight.

The spaces between the trabeculae allow room for blood vessels and nerves to flow through the bone. As well, in some bones, these cavities contain marrow – sites for the production of the red cells, white cells and platelets.

Fig 2 The vertical (VT) and transverse (TT) trabeculae, seen in white, are the struts of the vertebral body that prevent its collapse when loaded. Bogduk, 1997, page 7

Fig 3 A: With just an outer shell of cortical bone, the vertebral body would collapse, once loaded, like an empty cardboard box. C: Internal vertical struts brace the box. E: Transverse connections develop tension when a load is applied and keep the vertical struts from bowing. Bogduk, 1997,page 7

This combination of outer, dense cortical bone and inner cancellous bone (a mesh of branching bony struts and hollows) is such an effective model for the construction of bone that