First, the cotylo-sacral arch, on which the greatest number and force of shocks falls, is not placed vertically, but oblique& upwards and backwards, while the cotylo-pubic arch, being united to it at its extremities, and con tinued in the same plane over the femoral supports, forms the anterior arm of a bent lever of the first order, of which the cotylo sacral arch is the posterior curved arm, the spinal column the weight, and the heads of the femurs the fulcrum (see figs. 87. and 86.0.
The anterior or pubic arm of this lever giving insertion to the powerful extensor muscles of the thigh, mhich represent the power, is thrown upwards by the operation of downward force on the crown of the cotylo-sacral arch, calling these muscles into contractile reaction, which overcomes gradu ally the force of any shock operating at the posterior extremity of the pelvic lever over the fulcrum of the thigh bones. In well formed male pelves, the pubic arm of this lever is increased in power by being longer than the cotylo-sacral by / or of an inch, the one being 2 inches, the other 11, inch, in direct distance from the centre of the cotyloid support (fig. 87. a' b', a' c').. This gives the anterior muscles of the thigh greater power in resisting the downward force of the trunk at the sacral extremity of the lever.
But by the addition of the iliac and sacral projections posteriorly, the cotylo-sacral arm is increased in length by 2f inches, as will be observed on reference to the figure (c', d'), a disposition which evidently increases the power of the glutei muscles in maintaining the habitual erect position, and resisting any tendency to fall forwards, by extending the femora on the pelvis.
Ag,ain, if we follow the lateral curve of the sacrum at the brim of the pelvis, we shall find that it projects forward in the promontory of the sacrum, immediately under the point supporting the spine, so that its profile, taken with that of the ilium, as seen in the figure (e f), presents a curve with the concavity for ward, in the manner of a C spring. It is worthy of remark, that, in the erect position, a plumb line, dropped opposite to the centres of the bodies of the axis and last lumbar ver tebra, passes across the centre of the sacral promontory, and directly between the centres of the cotyloid cavities, as was proved by the experirnents of Weber. Such a plumb line marks exactly the line of the transverse vertical plane of the spinal column before mentioned, which, when continued down wards, passes through the sacral extremity of the pelvic lever, and also through the cotyloid fulcrum, dividing equally and vertically the heads of the thigh bones, and crossing the ischio-pubic rami about their suture (a a').
Thus, the oblique C-like curve of the cotylo sacral arch, or posterior bent arrn of the lever, meets this plane at its two extremities, di recting its concavity towards it like the arc of a circle to its chord ; and contributes, by its elastic reaction, to break the force of shocks operating through the spine and fe mora. In deformed pelves, we generally find that the sinking of the sacrum, the crown of the pelvic arch, under the weight of the trunk, produces an increased curvature of the iliac bones forwards by the 3 ielding of the C spring, and thus still further encroaches upon the dimensions of the pelvic brim.
The above considerations will illustrate the fallacy of the deduction of Cruveilhier in re spect to the statics of the pelvis. This cele brated anatomist remarks, that " the arti culation of the vertebral column with the pelvis is situated at the back part of that cavity, while the articulation of the femurs is anterior and lateral. The distance between them increases the space in which the centre of gravity can oscillate, without being carried so far forward as to pass beyond a perpen dicular, from the cotyloid cavity to the base of support at the feet " (p. 514,. vol. i. Anat. Descrz:p.). Now these experiments of Weber prove that the centre of gravity is directly ever the cotyloid support, and cannot oscil late between these two articulations. The only oscillation of the line of gravity which can take place without falling is along the length of the basis of support — the feet.
Soon after, in reference to the sitting po sition, he says, " the tuberosities of the ischia being a little anterior to the cotyloid cavities, and near the front of the pelvis, the centre of gravity tends to pass behind the base of support; and the body easily falls backward in that position." Now, the tuberosities of the ischia, in the erect posture, are consider ably behind the line of gravity, or transverse vertical plane, which crosses at or near the ischio-pubic ramal suture ; and though, in the sitting posture, they are brought a little nearer the line of gravity, yet a much more satisfactory reason of the trunk more easily falling backwards than forwards, is because of the support of the hams in front, and the eleva tion of the coccyx behind above the plane of support.