But ,these ligatnents, from their oblique direction inwards, at the same time that they resist downward pressure, pull with a corresponding force the sacrum and the ilia more closely together, and render, by this constantly tightening and bracing process, all the before-mentioned provisions for resisting displacement more effective, and, by a gradu ally increasing resistance, overcotne the impel ling force. To illustrate this effect, it may be mentioned that the effect of placing too much weight on the crown of an artificial arch is to cause the line of pressure (c, a, c,fig. 90. A), which ought to pass through the centres of the " voussoirs " perpendicular to their joints, to rise above the " extrados" at the apex, a, and to be brought within the inner surface or " intrados" of the arch, b, on each side; and this causes the voussoirs, a d, d c, to turn on each other at the edges nearest the line of pressure ; and in consequence the crown of the arch sinks and opens below, b, and the haunches rise and open above, d, d. The sacro-iliac joints, being the haunches of the cotylo-sacral arch, have in like manner a tendency to separate above and behind when the pressure on the sacrum is increased ; and this tendency is counteracted by the strong posterior sacro-ffiac ligaments. By the law of the resolution of forces, this tightening ac tion of the sacro-iliac ligaments may be ex pressed by the opposite sides, a b, c d, of a parallelogram (fig. n), of which the line of direction of the ligaments, b, c, forms the dia gonal, and the remaining sides, a c, b d, the sustaining power.
Lastly. Because of the oblique position of the sacrum with regard to the ilia, forces acting on the lumbar vertebrm have a ten dency to throw the base or sacral promon tory downwards, and to tilt the apex with the coccyx upwards, as is seen in the experiment of striking the separated extremity of the lumbar vertebrm before alluded to, by the im pulse felt at the sacral promontory. It will be better understood by reference to the drawing and diagram of a model made to re present the action (fig. 91. A. and n). • The tendency of the sacrum is to turn round the axis of the sacro-iliac joints in the curve d f e, ( fzg.88. A.), and round the centre a (fig. 91. c'). To counteract this tendency of the base downwards and forwards, the strong ilio-lumbar ligaments (a) pass backwards and outwards from the last lumbar vertebra to the crest of the ilium, upon which it obtains a long and broad hold. And to resist the tilting upwards of the apex of the sacrum, are attached the extremely powerful sacro-sciatic ligaments (b), which aid also the oblique sacro-iliac ligaments to resist backward displacement.
Thus are constituted two strong yet elastic springs on each side acting upon the con cavities of the lumbar and sacral curves, which have, perhaps, the most powerful influence of any that have been before mentioned, in breaking the force of shocks and concussions passing along the bones of the trunk and lower extremities. The importance of this office of the sacro-sciatic ligaments is seen in their great strength, and in the consolidation of the lower sacral vertebrm to which they are attached. The forward direction of the base
of the sacral wedge when taken antero posteriorly, as seen by looking from above, facilitates this elastic yielding of the sacral spring, as it evidently could not take place if the double sacral wedge had a small diameter directed forwards as well as downwards, in which case it would be prevented by the ilia from moving in that direction at all.
Thus the pelvic supports of the trunk are a peculiar and admirable combination of the arch and the suspension bridge. Under heavy weights, the preparatory tension of the pelvic muscles, as the psom, pyrifonnes, anti great glutei, will, by more closely approximating the sacrum and ilia, produce the conditions of the arch. But in sudden shocks, the strain will fall more immediately upon the ligamentous suspensory structures, as the sacro-iliac, sacro sciatic, and ilio-lumbar ligaments, more calcu lated, by their resiliency, to break their force gradually, and filially overcome them.
The thick, strong, and elastic fibro-cartilagin ous pads inserted between the opposing os seous surfaces of the sacro-iliac and pubic symphyses may be mentioned also, as con tributing to deaden and break the force of shocks passing through the pelvic arches ; and these being generally, as we have seen, arranged in two layers allowing of limited sliding motion between them, are better calculated to resist sudden shocks passing obliquely than single discs, such as the vertebral, which are chiefly disposed to resist pressure passing directly.
The posterior projection of the iliac tubero sities protects the sacrum, deeply situated be tween them, from direct force tending to produce anterior dislocation, while their in ternal direction prevents that bone from slip ping backward between them.
The great breadth and size of the sacrum in the human pelvis — it being proportionally larger than that of any other animal — indi cate its importance as the basis of support to the spine, and the crown of the pelvic arch ; and, in connection with the admirable mecha nical and architectural arrangements just de scribed, present a wide contrast to the pelvic structures of animals ; and prove the erect position to have been designed for the habi tual expression of the dignity of man.
The thigh of man, when standing, forms one line with the trunk, and makes an obtuse angle with the posterior arm of the pelvic lever ; but in quadrupeds it is directed much more for wards, and forms an acute angle.with both the ilia and the spine. In quadrupeds, the thighs are much closer together and more pressed upon the flanks, and, even when they rest on their haunches, they naturally support themselves on their fore legs. This is even seen to a great extent in the apes and monkeys, so difficult is it for them to maintain the centre of gravity in an erect posture. The extensors and flexors of the thigh on the pelvis are also more developed in man, in order to snstain with more firmness the erect posture. Hence the greater breadth of the hip and buttock, and the bulk of the thigh. The breadth of the pelvis also gives a greater leverage to these powerful muscles.