Instead of abutments to prevent the extre mities of the arch starting outwarls, we often see, in artificial constructions, a connecting link or tie extending between these extremi ties to hold them together, or the circle of which the arch is a segment is completed below, as in tunneling.
Such a tie and completion of the circle we have in the horizontal rami of the pubic bones, for the cotylo-sacral arch, and in the united ischio-pubic rami, for the ischio-sacral arch ; and they are connected in front, at the pubic symphysis, exactly as these two arches themselves are behind at the sacro-iliac joints.
By the vertical ischio-pubic arch thus formed, that portion of the pressure which has a tend ency to push forward and upward the extre mities of the ischio-sacral arch, is supported and thrown upon the cotylo-sacral arch, the whole weight of the trunk, in a sitting posture, being thus divided between them The ischio pubic rami are the parts of the pelvis most liable to fracture, according to Cruveilhier, from the application of force acting on the ischia.
The cotylo-pubic arch not only resists the starting outward of the ends of the cotylo sacral arch, but it resists their displacement inwards, which would result from the pressure of the femora in the direction of the necks of these bones. The effect of this pressure, when the pubes yield to it, is shown in the de formity which has been termed the rostrated pelvis, resulting from the crushing of these bones together. The cotylo-pubic arch also receives, in its concavity, part or the weight of the abdominal viscera, though, from the attachment of these to the spine, their chief weight is concentrated upon the common centre of pelvic arches,— the sacrum. The ilia are also generally supposed to support the intestines in a great measure ; but this support, on account of their great obliquity in the erect position, cannot be so important as is commonly imagined, except, as in the case of the ccecum and rectum, through peritoneal attachments. The human pelvis, when thus taken ,in conjunction with the thorax, forms the base of a cone, the apex of which is the neck, a disposition for supporting the contained viscera which the erect position demands, and which contrasts strongly with the structure of quadrupeds.
Again, the cotylo-sacral and pubic arches on each side, united at their extremities in the acetabula, form two lateral arches, on the centres of which rest the thigh bones. Against the lateral pressure exercised by the thigh bones, these two arches, connected, at their anterior and posterior extremities, by the symphysis pubis and sacrum, form, as Mayo observes, an elastic hoop. The ischia also contribute to this resistance against lateral pressure, and form, with the two other por tions of the innominate bones, a sort of arched tripod, on the apex of which the femur is supported.
In addition to the buttresses already de scribed, there is, placed vertically above the cotyloid cavity, a thick rib of bone, which transmits to the arched crest of the ilium, and through it and the sacro-iliac joint to the sacrutn, a portion of the direct vertical pressure from the heads of the thigh bones. This thickened portion of the iliac wing has been mentioned in the general description of the bone as impinging on the iliac crest in the middle of its anterior curve. The division of the pressure thus produced, no doubt calls into action much more com pletely the elastic resistance of the pelvis, in sudden increase of weight. Thus in the sacro-iliac joint meet three buttresses or thickened lines of pressure, of which the direct cotylo-sarral is the central and principal one, the ischio-sacral the lowest and next in strength, and the superior or indirect cotylo • sacral the weakest.
But, besides merely supporting quiescent superincumbent weight, the pelvic arches are required to resist and break the force of shocks and concussions meeting with the inertia of the trunk, and passing from the lower extremities of the body to the vital and delicate cranial and thoracic structures. These dynamic requirements are met by pe culiar modifications of the simple arch, com bining with it, by an admirable adaptation, the qualities of an elastic spring.