ture_ is by far the commonest and most
important deviation of the spine. The student will obtain a clearer
conception of the nature of this deformity if we consider in the first
place those types for which an obvious explanation is available.

_Static scoliosis_, for example, when one leg is shorter than the
other, the pelvis is tilted down on the short side, the
thoracico-lumbar spine deviates laterally to the normal side, and to
restore the equilibrium of the trunk the cervico-thoracic spine
deviates again in the opposite direction. The causes of one leg being
shorter than the other are numerous and varied; they include such
conditions as unilateral congenital dislocation of the hip, fractures
united with overriding of the fragments, diseases of the joints,
_e.g._, hip disease, or of the bones, especially such as interfere
with the function of ossifying junctions; and acquired deformities
such as unilateral flat-foot, knock-knee, or bow-leg. Clinically,
this type of scoliosis is identified by observing that when the
patient sits down the deviation of the spine disappears; it is
relieved or got rid of by raising the sole and the heel of the boot on
the short side, and, if required, by inserting an "elevator" inside
the boot.

When there is _shortening of the muscles on one side of the trunk_
there develops a lateral curvature of the spine with its convexity to
the normal side; a good example of this is afforded in cases of
infantile hemiplegia (Fig. 224) in which the deviation affects the
entire column: a localised form is seen in congenital wry-neck, in
which the convexity of the cervico-dorsal curve is on the side of the
normal sterno-mastoid with a compensatory deviation to the opposite
side in the spine below (Fig. 272). _Unilateral paralysis_ of
_muscles_ acting on the trunk may also cause a lateral deviation of
the spine, as is well seen in paralysis of the trapezius, which
results in a cervical scoliosis with the convexity to the
non-paralysed side.

[Illustration: FIG. 224.--Scoliosis following upon Poliomyelitis
affecting right arm and leg.

(Mr. D. M. Greig's case.)]

_Asymmetry of the thorax_, such as may follow on empyema with
defective expansion of the lung, causes a lateral deviation of the
dorsal spine with the convexity towards the normal side.

_Attitudes_ adopted to relieve pain, such as that caused by sciatica,
sacro-iliac or hip disease, in which the weight of the body is
transferred to the