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Explanation of Chart No. 1
FIGURE 1 represents a normal sacrum with a horizontal line across the surface of its base. This is an inverted triangle and serves as the level weight bearing line of the entire spinal column and its structure. The line is important and has to do with tilts, laterality and inferiority of the base as a whole or on one side.
FIGURE 2 shows the sacrum and spinal column intact, with a vertical line plus the horizontal one, which have become diagonal lines on the chart to bring out the effect of a lateral tilt more clearly.
FIGURE 3 is a drawing of the articulating surface of the sacrum with the innominate [hip] bone. The lines of stress are clearly given as a cross through the center of this articulation. This line has to do with anteriority, posteriority and twists of the sacrum between the innominates [hips].
The sacrum is a key wedge between the two innominate [hip] bones, like the keystone in an arch, between the two pillars on which this arch rests. It is the most vital structure in its position and effect in this part of the body.
FIGURE 4 portrays the sacrum and the spine with two stress lines which indicate its anterior or posterior position.
With this clear picture of the normal sacrum in mind and the simple lines of normal positioning, we can proceed to an over-all view of this norm in relation to the whole human body. It is necessary for the trained eye of the good doctor to see this relationship of the patient against some fixed background by which he can detect abnormalities. Even in taking x-ray pictures in the upright position, this precise arrangement, to the lines of gravity, is very important.
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