So, what then is the cranial index? As seen in the diagram below, the cranial index is the unit formed from the base of the skull (mastoid process) to the top of the skull. This unit becomes a box which can then be rotated in space.
If we accept the cranial index as the unit of measurement for human proportion, then the beauty of this system soon begins to unfold. For example, if we look at the distance from the mastoid process to the suprasternal notch, also called the jugular notch, we find that it is one cranial index unit (hereafter labeled “CI”). Or, if we look at the width of one clavicle, we find that it is one CI. Or, the length of the sternum = one CI. Or, the width of the scapula = one CI.
If we consider the rib cage, we again find an intriguing correlation. The rib cage is four CIs:
Fortunately, it doesn’t stop here. Let’s now consider the arms and the pelvis. The upper arm is two CIs long, the lower arm to the tips of the metacarpals is two CIs long, and the width of the pelvis is two CIs.
And it carries through with the legs as well. The upper leg is three CIs long and the lower leg plus the foot is also three CIs.
Now we can see how this idea looks superimposed on the skeleton as a whole:
Looking at these diagrams, we can see that the power of using the cranial index as a unit of measurement is that it frequently intersects with the bony landmarks of the skeleton, which are fixed, as opposed to fleshy landmarks (like navels), which are not. For this reason, I find this approach much more helpful than trying to measure how many heads high a figure is. When you use the cranial index, you know that the relationships you observe will relate to the bones, which is where it all starts.
I would like to thank Michael Mentler for allowing me to use images he has created at his studio, www.tsofa.com.