The deformities of the pelvis in curvature of the spine are, in many respects, still more remarkable, and an acquaintance with them is, at the same time, more important. They are frequently the primary deformity, and the spinal curvature is the consecutive; but even independently of these cases, they are not, as Meckel asserts, uncommon: on the contrary, they are so frequent, that it may be regarded as an exception to find a completely normal pelvis where there is any curvature of the spine.

In the great majority of cases the following deviations may be recognized with tolerable distinctness, and they may all be explained according to fixed rules.

The pelvis is always oblique in lateral curvature of the spine, and exhibits a want of symmetry in its two halves that is sometimes striking. The half of the pelvis on the side opposite to the upper or dorsal curve is higher than the other; the extremity connected with it appears shortened; that is to say, the several parts of the two limbs being of equal length, and the necks of the two femurs placed on a level with each other, the trochanter, the knee, and the heel on that side are higher than those on the other side: the elevated half of the pelvis inclines less than it should, and is, at the same time, narrow; the transverse diameter of the inlet, therefore, is greater than natural. The circumstances from which these changes appear to arise are twofold. In one respect they are due to a change in the position of the sacrum, which is both curved and rotated on its axis towards the side opposite to the lumbar curve. The consequence of the former - the curvature - is, that the os innominatum is somewhat lifted at the sacro-iliac joint; and the latter - the rotation - carries the posterior part of the bone into the cavity of the pelvis: the adjoining portion of the ilium follows; but, as the innominatum is fixed at the symphysis pubis, the linea arcuata bends near the sacro-iliac joint, and thus diminishes the capacity of this half of the pelvis in its antero-posterior diameter, at the same time elongating its transverse dimensions. But further, it is sometimes perceptible at the first glance, that the narrowness of this half of the pelvis is partly due to the flattening of so much of it as is anterior to the acetabulum. The reason of this flattening is found in the lumbar curve transmitting the weight of the body principally to the limb of that side. The linea arcuata then stretches in a straighter course from the angle already formed in it to the pubes, and the distance between the ilio-pec-tineal eminence and the promontory is diminished.

When, in consequence of combined lateral and angular curvature (Scoliosis kyphotica) in the dorsal region, the compensatory lateral curvature of the loins bends forward to a corresponding degree, the dimensions of the pelvis are narrowed still more; for the promontory, on the one hand, projects further into the upper aperture on the side of the curve, and as, on the other, the weight of the body is still more directly transferred to the neck of the femur on that side, the ilio-pecti-neal eminence is pressed inward and upward to a greater extent.

Observation of the character of the supplemental curvatures, therefore, enables us to predicate the deformity of the pelvis which exists in lateral curvature of the spine: the dimensions of its cavity are altered on the side opposite to the upper or dorsal projection, or on the same side as that to which the lower, or lumbar curve, or the spinous processes of the upper curve are directed.

Now and then, however, there are exceptions to this rule. Some very decided lateral curvatures of the dorsal region, which project considerably beyond the centre of gravity of the body, are continuous with a lumbar column which is, proportionally, much less curved; and the weight of the body, therefore, is transferred to that side of the pelvis which the dorsal hump overhangs. The innominatum on that side is then the higher, although the other changes which result in narrowing of the pelvis are still found, as in ordinary cases, on the side opposite to the dorsal curve. When beneath a lateral curvature to the right, in the upper part of the back, there is a curve in the lower dorsal and upper lumbar region to the left; and this is succeeded by a third deviation of the column in the lower part of the loins to the right, while the sacral vertebrae swerve again to the left; and when in such a case the second curve is greater than the rest, the left half of the pelvis will bear more of the weight of the trunk, and will stand higher, and have less inclination, than the right; while the deviation of the sacrum will give rise to the usual narrowing of the pelvis on the right side.

The pelvis, in angular curvature, is generally very capacious; its height is considerable, and the predominant diameter is manifestly the conjugate; the inclination varies, but usually - that is, when the projection occupies the usual situation in the lower dorsal, and adjoining lumbar regions - it is very decided.

This conformation of the pelvis chiefly arises from the diminution in the size of the abdominal cavity, which is produced by the depression of the thorax: the difference in its inclination depends especially upon the extent to which the compensatory curve projects forward. For the angular projection consists of two legs which diverge, under varying angles, from each other, one upward and the other downward. The greater part of the compensatory incurvation falls to the one or to the other of these legs, according to the situation of the projection; and the inclination of the pelvis varies with the distribution of the duty of compensation.

If the angle be situated in the lumbar region, the small remainder of the column in the loins, which forms its lower leg, is insufficient to counterbalance the projection backward, and there is no need of any further deviation in the pelvis, for the upper leg, which runs up to the dorsal part of the column, undertakes the compensation, and curves gently forwards. The inclination of the pelvis is then nearly natural.

The lower leg of an angular projection, situated in the inferior dorsal region, is longer and needs a more decided receding of the sacrum for its support. The inclination of the pelvis is then greater than natural.

When the angular projection is situated high in the dorsal region it is counterbalanced principally by curvature of its lower leg; and the sweep forward in the dorsal and lumbar region necessitates a still more marked inclination of the pelvis.

But, when the projection is in the lowest part of the lumbar region, the state of the pelvis is just the opposite. The rest of the vertebral column then forms the upper leg of the angle, the sacrum alone forms the lower. The pelvis is raised to a degree corresponding with the size of the angle, and the level of its upper aperture may even become horizontal; its inclination is annihilated, and the projection can be counterbalanced only by anterior curvature of the spinal column. If the number of vertebral bodies destroyed be considerable, it results as well from the loss of substance as from the elevation of the pelvis, that the trunk is shortened, and its two cavities narrowed; and a further consequence is, that the thorax acquires a peculiar shape. In the trunk of a woman, of 34 years of age, who died in childbirth, from rupture of the uterus, and who had had a difficult labor five years before, the spine was found projecting at an obtuse angle, in consequence of carious destruction of the bodies of the four lower lumbar vertebrae. The dorsal portion of the column from that point upwards, described a slight curve arching downward, while the sacrum, very flat and straightened, and constituting the lower leg of the angle, had raised the pelvis so much, that it had lost its inclination almost entirely, and that the distance of the upper margin of the symphysis pubis from the ensiform cartilage, was scarcely 3f inches. The antero-posterior diameter of the thorax was much contracted (compare page 187), so that the ensiform cartilage was not more than 3½ inches distant from the lower margin of the eighth dorsal vertebra, which was over against it: the transverse diameter, however, measured more than 9½ inches. The reason of this was that as the upper leg of the angle could alone undertake the compensation, the abdomen, being exceedingly narrowed from above by the anterior curvature of the column, and from below by the elevation of the pelvis, was thrust forward, and became pendulous, and the thorax was flattened from before backward, as in an ordinary anterior curvature in the lumbar region, by the same action of the abdominal muscles as resisted the expansion and sinking of the abdomen. It is worthy of notice that, without a close examination, so pendulous an abdomen might lead to the inference that the inclination of the pelvis was increased.

The ordinary anterior curvature in the loins involves a corresponding displacement of the pelvis backward, that is to say, excessive inclination of it; and more especially when the lumbar curve is itself compensatory and consequent upon too great inclination of the pelvis. When it arises from rickets it is always associated with diminution of the conjugate diameter of the pelvis: and when it is combined with lateral curvature also, the two halves of the pelvis are unsymmetrical.

But in some cases in which the lumbar vertebrae sweep forward into the pelvis, not only is there no increase in the inclination of the pelvis, but on the contrary, there is scarcely any, or none at all. Such cases are proved by what has been remarked already, and by the cause from which they generally arise, to be instances of angular projection in the lowest part of the lumbar region, which are compensated by anterior curvature of the upper leg of the angle.