MECIIANICS OF THE HUMAN PELVIS.
When we consider the pelvis with regard to its architectural adaptations, and compare it with the principles of engineering, we are struck with the beautiful simplicity of the means by which it combines strength with elasticity, and lightness with capacity and unity of design. The weight of the trunk is to be transmitted through the lumbar ver tebrae to the sacrum, arid from thence to points of support, which vary with the posi tion of the inferior extremities. In the erect position, these points are the femora. ln the sitting position they are the tuberosities of the ischia.
The experiments of Weber have proved that though the centre of gravity of the trunk itself (without the legs) is placed in the transverse vertical plane as high as the sterno-xiphoid joint, yet the centre of gravity of the whole body, as marked by the point of section of the before-mentioned transverse vertical with a horizontal plane, is placed only 8•7 millimetres above the sacro-lumbar joint, or just above the pelvic arch. All weight on the arch, such as that of the trunk, is sup ported most easily when its line of gravity falls through this part, and coincides with that of the whole body in the transverse vertical plane, and the trunk will be found to be thrown into such a position, when supporting heavy weights, as will tend to produce this effect. And, according to the researches of Rokitanski, when the sacro-vertebral angle and pelvic inclination is increased, as by hip disease, there is always a compensatory curve of the spine backward, to keep the centre of gravity above this point.
In artificial constructions for the purpose of transmitting a weight downwards from a central to two lateral points, a segment of a circle, or arch is most commonly made use of; and generally consists of two lateral curved portions, composed of separate parts or voussoirs, with an interposed substance between them called a keystone or crown piece, of a wedge shape, and placed with the broad end uppermost. This wedge shape exactly corresponds to the interval which would be left between the lateral curved pieces, having their ends cut square. Any
force operating on the keystone from above tends only to drive the broader part of the wedge further between the lateral pieces, at the satne time pressing them nearer to each other, and so to increase the firmness of the arch, so long as the extremities are firmly fixed in the ground and prevented from start ing outwards, which is generally accomplished by means of abutments.
Constructed upon this plan, the pelvis pre sents two lateral curved thickened buttresses or columns, passing from the cotyloid to the sacro-iliac articulations, and two others pass ing on a plane posterior to these from the tuberosities of the ischia along their de scending rami, and through the ilia to the sanie sacro-iliac articulations; and interposed between each of these corresponding lateral pieces is the common keystone of both the arches thus formed, the wedge-shaped sacrum.
The sacrum thus forms the common cul minating point of two arches, viz., the cotylo sacral or standing arch, and the ischio-sacral or sitting arch. And the planes of these two arches are so directed as to coincide or be come applied to each other at the top of the great sciatic notch, as may be seen by reference to the diagram (fig. 86. A, a a' ). In this comparison, however, it must be borne in mind, that the extreme tenacity and strength of the material used, bone, obviates the ne cessity of the use of many pieces in the lateral portions, such as the " voussoirs" of stone arches, and is more analogous to the iron materials sometimes used for this purpose. And it is in these lines of pressure that we find the bulk and strength of the osseous structure of the pelvis most displayed.
The span of the cotylo-sacral arch being greater, and more elliptical in the female than in the male, where it is altno,t circular, renders them less able to support heavy weights ; and on account of the greater distance to which it separates the femurs, contributes to produce the waddling gait in running which is characteristic of this method of female pro gression.