As we saw in the last post, it is well known that the skull is unevenly balanced at the atlanto-occipital joint, and that the balance of the skull plays a critical role in posture. If, for instance, the head is pulled back, this tends to exert a downward force on the spine, making it difficult to maintain upright posture.
But exactly how do we know that the head is weighted forward on the spine, and why is this important to upright posture? If we observe the human skull, it is not difficult to see that the majority of its bone mass is forward of the atlantooccipital ("A-O") joint. The face and jaw are there, and the underside of the skull in front of the A-O joint is more complex in shape than that behind it. If we were to precisely measure this bone mass we would find a point in the skull where, if we divided it in half there along any plane, there would be equal mass in both halves. This point is called the center of gravity. The skull’s center of gravity is usually said to be located at the sella turcica, a familiar landmark at the base of the skull above and forward of the A-O joint (the red circle in the illustration to the left).
We can determine the location of the skull's center of gravity accurately using a simple experimental method: we suspend one from a point on its surface by a length of string. If we trace an imaginary line – a “plumb line” -- from that point straight to the ground, and then repeat this process from any other point on the skull, we find that all the plumb lines intersect at a single location. This point is the center of gravity.
It can be difficult to attach string to a skull. Helpfully, the skull has many small holes on its surface. If we tie a large enough knot in the string, we can thread it through one of the holes and the knot will catch. This serves as a suspension point for the demonstration. There are ocular holes, cranial nerve holes at the base, gaps between teeth, and in some model skulls there is a hole at the top of the cranium for a securing rod. Any of these will do. The photograph to the right shows a model skull suspended by string from the cranium.
Once suspended, trace the plumb line along the outer surface of the skull, as shown in the photograph to the left. Then open the skull up and trace along the inner surface, as shown in the photograph to the right.
When we repeat this process, suspending the skull from other points and tracing the plumb lines, we find the point of convergence is indeed at the rear edge of the sella turcica, on the inner surface of the skull.
Viewed from the side, the center of gravity lies on a plane just above the rear edge of the cheek bone, known as the zygomatic arch. Viewed from the face, it lies on a plane at the top edge of the nasal bone, just under the brow ridge.
On the model skull we have used, the center of gravity is about an inch forward and an inch above the atlantooccipital joint, or about 1.5 inches directly away.
In the next piece, we will discuss what all this implies about how the skull balances on the spine.