Again, by the projection of the sacrum and tuberosities of the ilia behind the sacro-iliac joints, another lever, less powerful than the fOregoing, is formed, having also the cotylo femoral supports for its fulcrum, and the spinal column for its weight, the anterior arm of this lever being the cotylo-sacral arch (fig. 87. a', c'), and the posterior, the over hanging tuberosities of the ilia and projecting sacrum (ci, d'). Measuring from the centre of the sacro-iliac articulation, the anterior arm is If inch in direct length, and the posterior about 2 inches and a half. The power in this lever resides in the power ful muscles which pass from the sacral and iliac bones posterior to the sacro-iliac joint, to the osseous spinal projections and append ages above--viz. the longissimus dorsi and sacro-lumbalis, and its action is shown in the increase of the pelvic inclination on the change from the sitting to the standing position ; the principal movement taking place in the sacro lumbar joint. It acts to the most advantage when the centre of gravity of the trunk, from which it is derived, is thrown in advance of the cotyloid fulcrum (a a), so that the lever, though apparently one of the second order (i. e. in which r and w are on the same side of the fulcrum), is in reality one of the first order, 1in which the fulcrum may be between them, and supports both the power, P, and the weight, w. Hence, in the drooping of the trunk forwards in old age, the action of these muscles contributes, to produce the in creased obliquity of the pelvic lever in the manner before described. This may be made more evident by inspection of the diagram (fig. 86. n), which is taken from a small brass model made to illustrate this point. It will be seen that the pressure on the cotyloid fulcrum, F, could not be w—P, as in the second order of.levers, but must necessarily be IV+ P, and therefore in the first order of levers. P, in this case, may be represented by the line P c, and the line of gravity of the trunk, c a, and is doubtless considerably increased by the resisting tension of the an terior abdominal muscles acting through the extensors of the thigh on the femoral sup ports in the line c b, b d.
In the foregoing dispositions of the pelvic structure, the office fulfilled by the sacruna is so compendious and important as to call for particular attention to the shape and position of that bone, and the manner in which it is articulated with the ilia, so as to be at once firm and strong as a keystone, yielding as a spring, and moveable as a jointed bone. And we shall find all these requirements beauti fully provided for.
First. The sacrum is wedge-shaped, when viewed at its anterior aspect, narrowing from above downward, especially along the surface immediately between the lower portions of its iliac articular surfaces in the plane b, e (fig. 88. A), which are inclined to each other at an angle varying from 15° to 30°, and aver aging about 20°. When viewed from above at its base, as in fig. 89. page 144.), it also pre sents a wedge shape with the base directed ante riorly, the lateral masses of the base becoming narrower from before backwards ; so that the sacrum appears to be a double wedge, having its broadest part at the border of junction of the base with the anterior surface, and tapering from this point, both upwards and backwards and downwards and backwards.
Hence it has been stated by Cruveilhier to be liable to dislocation downwards and for wards, from the want of bony support in that direction. But the sacrum, in the erect po
sition of the body, is placed, not vertically, but obliquely, with its base directed more forwards than upwards, and its anterior sur face more downwards than forwards, so that the upper limb of the auricular surface is placed nearly vertically, and the lower nearly horizontally, as seen in fig. 88. A. The di minished breadth of the base of the sacrum posteriorly is due to the bevelling of the lateral surfaces for the implantation of the deep posterior and interosseous ligaments, as seen in fig. 88. (n, e), the bone not being here in apposition with the overhanging iliac tuber osities, — the area of absolute contact being confined to the auricular surfaces themselves. Again, the increased breadth of the anterior surface at the auricular angle c (fig. 89.), will be found, on careful inspection, to depend upon the presence of the pointed projections on each side, before described as received into corresponding depressions on the au ricular surfaces of the ilia, which latter, being circumscribed below by a raised border, cause the sacrum to bite on the ilia here to a consider able extent, forming what is called, in engi neering nomenclature, a " joggle" to the sacral " voussoir." By the iliac support thus re ceived, the position of the sacrum is well protected against pressures, coming either directly downwards in the line d,b (fig. 86.A), or downwards and forwards in the direction of the line a, b ; b being placed in the diagram upon the sacral projection. By measuring in eight sacra, the distances between the upper extremities of the auricular facets on each side, at the point d, marking them off on paper, and then taking the distances in like manner at c, which corresponds to the lateral notch opposite the second sacral vertebra (see fig. 78. A, b.), I found the line d, c to coincide pretty nearly with the mean direction of the superior vertical limb, and with the superior half of the central curved groove d, e ; and to fall in a plane which was inclined to the one on the opposite side, so as to form with it an angle varying from 15° to 25°, and in all cases directed downwards.
Now, the sacrum is so placed in regud to the cotylo-sacral arch, and the line of pressure froni above, that the angle formed by the surfaces of the base and anterior face is the narrowest point of a rapidly-expanding arti cular wedge placed antero-posteriorly. This is better seen by a lateral view of' the auricu lar facet, with the bone in the natural oblique position, as in fig. 88. A. The facet will then be seen to have its angular projection pointing downwards and forwards in the direction of the cotylo-sacral arch in the line a b, and its two limbs diverging so as to pre sent a broad surface of articulation with the ilia in the lines b d, b e, forming with d e, the trian,gular " voussoir " d b e. The depth of the keystone is the greatest distance between the anterior border of the superior limb, d, and the inferior extremity of in ferior limb, e (fig. a), and is about 21 inches in the adult male. The wedge shape formed by them is well seen in the posterior view of a transverse section along the sacral axis, as in fig. B, where the lines a c, b c show the obliquity of the wedge, and form an angle a c b, of from 20° to 35°. In a direction downwards and backwards, then, the sacrum has, like an artificial " voussoir," or keystone, its broad end directed upwards towards the point of pressure.