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 we find it employed throughout the body. The image below looks at the architecture of the thigh bone.

Fig 4 The use of outer cortical (compact) and inner cancellous bone is seen everywhere in the body. This is a vertical slice through the head, neck and upper portion of the shaft of the femur or thigh bone.

As we review this, remember there is no standard template for the laying down of these bony struts. They are established as the bone responds to the particular lines of force we decide to apply to them. It is our habits of posture and movement that create our bony architecture. As we live in our skeleton, so is it shaped.

The Taoist Tai Chi™ internal arts of health involve a weight-bearing exercise whose movements mimic the turning and lengthening that are meant to be a part of every day. These movements tug at all our bones and initiate the formation of new bone where strength is needed. Building strength while maintaining lightness – an elegant solution for an organism that must deal with gravity all its life.

Look at how the lines of force acting on a lumbar vertebra shape other parts of its internal architecture:

Fig 5 Lines of trabeculae bone laid down in (A) the vertebral body and spinous process, (B) facet joints and body, (C) the transverse processes, arch and body. Bogduk, 1997, page 10

Now, let’s turn to the vertebral arch – a bony ring that fuses with the posterior aspect of the vertebral body. Looking back at figure 1, we see that attached to the arch are:

• 4 facet joints. The 2 superior and 2 inferior facets allow the arch of one vertebra to articulate with the arch of the vertebra above and that of the one below
• a spinous process which projects posteriorly and is readily appreciated as you run your fingers down along the midline of someone’s back
• 2 transverse processes which project sideways. Covered by the muscles of the back, they are not apparent to the eye or hand. Both the spinous and transverse processes serve as attachment points for tendons (connect muscle to bone), ligaments (attach one bone to another) and fascia.

The vertebrae line up, sitting one on top of another. Their vertebral bodies are joined by the interposed intervertebral discs. The arches connect through the facet joints. As these multiple articulations of the vertebral column link together, the space between the bodies and arches creates the spinal canal. This continuous channel runs from the foramen magnum, the hole in the base of the skull through which the spinal cord passes, all the way down to, and into, the sacrum, providing a flexible, protected space for the nervous tissue of the spinal cord.

Fig 6 The five lumbar vertebrae. The arrow depicts the spinal canal. Bogduk, 1997, page 59

Fig 7 A view of the 3rd and 4th lumbar vertebrae from behind. Netter, 2006, Plate 155

Kapandji points out that bone and soft tissues alternate as you run down the spinal column. The bony elements (the vertebral bodies, the arches with their attachments) create passive segments. The soft tissues – the discs, facet joints and spinal ligaments – are active. It is the active segments, powered by the spinal muscles and ligaments, that permit movement of the spine and alterations in its curves.

Fig 8 Seen from the side, the passive segment (1) is made up of the vertebra itself - the body, the arch and its attachments. The active segment (11) consists of the intervertebral disc, intervertebral foramen, facet joints and spinal ligaments. Kapandji, volume 3, 2008, page 19

All these joints, articulations, moving parts of the spine provide flexibility to a structure that must, at the same time, maintain coherence and integrity.

In the next several posts, we will look at the facet joints and discs and how they contribute both mobility and stability.

1. Atlas of Human Anatomy, 4th Edition, Frank H. Netter, 2006, Saunders Elsevier, ISBN-10: 1-4160-3385-8

2. Clinical Anatomy of the Lumbar Spine and Sacrum, 3rd Edition, Nikolai Bogduk, 1997, Churchill Livingston Elsevier, ISBN 0 443 06014 2

3. The Physiology of the Joints, Volume Three, The Spinal Column, Pelvic Girdle And Head, A.I. Kapandji, 2008, Churchill Livingston Elsevier, ISBN-10: 0702029599

Bruce McFarlane, MD

© 2010 Taoist Tai Chi Society of Canada