The alteration of the position of the centre of gravity of the trunk, bi deformity of the spine low down, will also have its effect upon the pelvis, by necessitating a constrained and unnatural position to prevent the body falling. Deformities confined to the upper part of the spine are seldom accompanied by deformed pelvis, owing, probably, to the little effect they have in altering the centre of gravity. In addition to these general chmges from mechanical pressure, there is, in this softened state of the bones, the powerful effect of combined muscular action.
The influence of continued posture on these changes vtill be found to be the origin of most of the differences of form we have seen in pelvic distortions. Let us consider the effects of mechanical position and muscular action in the recumbent,— sitting,— and standing posi tions respectively, on the softened pelvis In lying upon the back, the softened pelvis ail! have a tendency to become flattened antero-posteriorly, by. the sinking of the pubic arch, at the same time that the traction of the femurs and muscles of the lower extremi ties outwards will tend to separate the aceta bula and increase the transverse diameters. This 1 apprehend to be the commencement of the elliptical pelvic deformity, whieh occurs in the majority of the softened pelves of infants, whose most frequent and long-continued po sition is the dorsal recumbent. The angles of the pelvis with the spine will also have a tendency in this posture to becotne in creased by the weight of the inferior extremi ties. If the softening be great, and the position long-continued, the symphysis pubis would also sink, producing the form of pelvis ; a disposition which would he increased by the traction of the levator ani and weight of the bladder. There would also be a tendency to flattening of the sacrum. In lying upon the side, on the other hand, there is a pressure, through the trochanters, upon the acetabula, which, if long and fre quently-accustomed, will cause the lateral pelvic arches to yield and bend inwards at the cotyloid line of junction, in children as yet unossified, and produce the first bend or tendency to the angular deformity. The effect of these first impressions are, as Dr. Ramsbotham observes, illustrated by making an elbow in a piece of wire subjected to pressure at each extremity. In the unde veloped pelvis also, the facility with which these impressions are made upon the pubic tie is rendered greater by the greater tardiness of its ossification than in the other innomi nate pieces. In some instances, pelves seem to have been impressed in this manner on one side only, so that the two sides present an approach to the two different varieties of de formity, as will be presently alluded to.
In the sitting and standing positions, a more powerful distorting influence is brought into play—viz. the pressure of the weight _of the body on the softened pelvic arches.
The sittin,„,.. posture, when the elliptical form has already been impressed upon the pelvis, will still further tend to separate the .acetabula by the starting outwards of the lower extremities of the ischio-sacral arch under the pressure of the weight of the trunk on the sacrum ; and thus the separation of the tuberosities, the enlargement of the transverse diameter of the outlet, and the spreading out of the sub pubic arch take place. At the same time the sacral promontory sinks into the pelvis under the weight of the trunk, while the lower part of the sacrum is kept forwards by the sciatic ligaments, so that a bend takes place in the middle of the bone. This bend will be still further increased by the divergence of the ischial tuberosities, permitting the weight of the spine to be brought to bear upon the coccyx and lower end of the sacrum and against the sitting surface. The total direc tion of the pressure on the ischial tuberosities being upwards and backwards, the curve of the Ischio-saeral arch (coinciding with that of the cotylo-sacral at tbe top of the sciatic notch) takes place in that direction, and in creases the acuteness of the C-like curve.
These effects upon the sacrum and ilia, and pelvis generally, will be increased by the action of the powerful erector spinm muscles, and psoas and iliacus muscles, exer cised in keeping the trunk erect upon the pelvic lever (see fig. 122 E, c d). These muscles have, in addition, much influence in shortening the spinal column itself, already bending under the weight of the body, and,— following the general tendency of elongated substances yielding to pressure at both ends to twist laterally,—the lumbar curve and sacral promontory become placed on one side the median line. This tendency, from reasons before explained, is generally to the left. Under the increased inclination of the pelvic angle, the abdominal muscles will tend to draw the flattened pubes upwards still nearer the sacral promontory, diminishing the con jugate diameter. In extreme deformity, the iliac wings are pressed still further outwards and everted by the pressure of the lower ribs resting upon them, as we have observed in one of the detailed examples.