This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
When, as in lateral curvature or scoliosis, there is a pathological curve developed, the centre of gravity is shifted from the midline to one side and it falls nearer the foot of the side toward which the trunk is inclined (see Fig. 496, A). This makes the equilibrium unstable so that to restore stability the hips are inclined to the opposite side and the centre of gravity is brought once more midway between the ankles (Fig. 496, B). This condition is produced when there is a single incomplete curve or deviation to one side; if, however, the curve is complete and again reaches the median line, as is often the case in scoliosis, then the centre of gravity is not disturbed and there is no lateral shifting of the pelvis (see Fig. 496, C). If the primary curve is accompanied by a secondary curve, both being complete and crossing the median line, then also there is no shifting of the pelvis (Fig. 496, D). If, however, the curves are so irregular as to shift more of the weight to one side than the other, then the pelvis shifts (Fig. 496, E). This causes the hip on the side opposite to the inclination to appear higher than the other, but it is not really so and the pelvis still remains level. It is therefore evident that it is unnecessary and unwise to attempt to correct the deformity by raising the apparently low hip by a high shoe. All these conditions occur in the lateral curvatures or scolioses of childhood and adolescence as well as the deviations which occur from empyema, sciatica, Pott's disease, and other affections. A knowledge of the principles involved is essential to com-prehending their production and to directing the exercises and applying the apparatus used in their correction.
Fig. 496. - Deviation of the spine above the sacrum.
Fig. A. - If the trunk is inclined to one side, a vertical line a-b through the centre of gravity is shifted to c-f, and therefore falls outside of the base of support d-e and unstable equilibrium results.
Fig. B. - The inclining of the trunk to the right has been compensated by shifting the pelvis to the left, and the vertical a-b through the centre of gravity c falls within the base of support d-e and stable equilibrium has again been restored.
Fig. C. - If the deviation of the lower part of the trunk to the left is counterbalanced by a deviation of the upper part to the right then the vertical a-b through the centre of gravity c falls within the base of support d-e and the body remains in stable equilibrium.
Fig. D. - If a complete curve in the lumbar region is compensated by a complete curve in the dorsal and cervical regions above, then the centre of gravity c is not shifted and a vertical line through it still falls within the base of support d-e, and the body remains in stable equilibrium.
Fig. E. - If the curves are irregular, shifting more of the weight of the upper part of the body to the right, the pelvis is shifted to the left until the centre of gravity c is again brought within the base of support d-e and stable equilibrium is again restored.