MECHANISM OF THE PRECEDING PEI. VIC DISTORTIONS.—In considering the forces which operate in producing the two principal varieties of pelvic distortion previously treated of, it is necessary carefully to separate those resulting from mechanical position, from those which arise from muscular action alone. In considering the former, it will be necessary as carefully to separate the idea of the line of gravity—i. e. a perpendicular line let fall front the centre of gravity— from that of the fine of pressure, whiel must necessarily pass through the osseous supporting structures, whatever disposition they. may have.
The centre of gravity of the trunk itself is that which influences most considerably the form of the softened pelvis in the sitting as well as the upright position. This is placed by Weber in the transverse vertical plane of the spinal column, which here falls consi derably in front of the vertebrx, in the tho racic cavity, at the level of the sterno-xiphoid articulation (seefig. 122. A, a).
In the most easy standing position, this centre of gravity is placed directly above that of the whole body, at the sacro-lumbar articu lation (A, c); so that perpendicular lines let fall from each to the ground will exactly co incide, and (in the well-made subject, after passing through the sacral promontory and the acetabula) fall between the feet as the basis of support. In the sitting posture, it falls a little posterior, between the ischial tu berosities.
To preserve the standing posture, it is necessary that the line of gravity of the whole body—viz. that from the lower centre—should fall anywhere between the extent of longitude of the feet (c d).
If the trunk, however, bend forward, its centre and line of gravity is advanced beyond that of the whole body, and a share of the sup port of the trunk, equivalent to the degree of distance of these two lines (b,d),falls upon the muscles and ligaments of the posterior part of the spine, and a corresponding strain upon their attachments to the sacrum and posterior part of the pelvis. This instance may be taken as an example of many others, in which the me chanical position of the line of gravity in fluences muscular action, the effects of botlt falling upon the lines of pressure and support in the pelvis.
For the preservation of the sitting position, it is only necessary that the line of gratity of the trunk (a b) should fall within the extent of the basis of support, which is, from more or less of the whole extent of the hams in front to the ischial tuberosities behind. Hence
the greater facility with which a person sitting down is pushed backward than forward.
The line of pressure, on the other hand, passing down the centre of the bodies of the vertebrw will, in the well-made subject, when standing on both legs or sitting, divide at the sacral promontory, into two equal parts, each of which traverses, first the lateral sacral masses to the sacro-iliac joint ; from this point, in the upright position, it passes along the cotylo-sacral rib to the heads of the femurs, describing in its course the C-like curve. In the sitting posture, it passes down the ischio-sacral buttress to the tuberosities. It will be borne in mind that each of these standing and sitting arches has its tie, which prevents it equally from starting outwards or pressing inwards at the extremities ; that for the cotylo-sacral arch being the united supe rior pubic rarni, and that for the ischio-sacral arch, the united isehio-pubic rami. The co tylo-sacral arch and its pubic tie, united at the acetabula, and placed in the same plane, form in man, as lite have seen, a lateral arch made up of the two halves, which supports on its cultn the inward pressure of the heaa of the femur. The cotylo-sacral portion also sustains, in addition, its upward and backward pressure.
The first effect of the softening of the os seous supports in this line of pressure is to increase the natural curves which occur in it. Thus we see an increase in the dorsal, lumbar, and sacral curves, in the cottlo-sacral, fe tnoral, and tibial (fig. 122. E). The next effect is to produce lateral curves, which present generally their concavities towards the line of gravity, and are always associated with com pensating curves, so as to keep the line of gravity within them, about which they pro duce a wavy line, as is seen in the deformed spine. When this is not the case, the support of the weight falls more upon the tension of the muscles, and ligaments, and parts of bone on the convex side. In the pelvis, and, to some degree, in the bones of the legs, however, these results are modified by the lateral duplication and division of the lines of pressure ; and in the pelvic skeleton this effect is still further increased by the cir cular union of the lateral structures, and by the pressure or traction or the bones of the legs, conjointly or individually.