But, as Cruveilhier has justly- observed, forces acting in the curve of the lumbar vertebrm are partly counteracted by the elastic spring-like yielding of the lumbar and sacro-lumbar fibro cartilages ; and by the lumbar curve they are, at the same time, di rected backwards a.s well as downwards (viz., — at first in the direction of the lined, f (fig. 88. A), and then in that of f,e, which latter, produced to meet the vertical line d, b, at g, forms an angle d g e, of about 117°, coinciding with the average sacro-vertebral angle in the direction of the sacral axis), —.thus tending to drive the broad end of the sacral " voussoir " between the narrower iliac intervals ; and so, in relation to the direction of the prin cipal forces acting on the pelvic arch, the sacrum becomes a true key stone.
Another arrangement which would tend, from the obliquity of the bone, to counteract any forward displacement, is the sudden in version of the vertical sacral wedge at the ex tremity of the lower limb of the auricular surface (fig. n,e), opposite the third sacral bone, at which point, we have mentioned in the description of the sacrum, the anterior surface becomes suddenly broader from above downwards; so that here the sacrum by an other " joggle" again bites on the iliac. A third disposition preventive of this displace ment has been pointed out by Mr. Ward, in the superiority of breadth of the posterior over the anterior surface of the sacrum, op posite the point c. (fig. A.), the middle of the inferior articular limb in many instances.
Behind and above the angular projection on the sacral facet is an elongated depression or groove, which passes along the centre of both limbs of the auricular surface, and re ceives a 'reciprocal elevation on the iliac articular surfitce (fig. 89.f.) Now this ridge on the iliac surface evidently bites in its turn on the sacrum, and presents another obstacle to anterior displacement in the superior limb, as well as to downward displacement at the inferior limb. The surfaces thus applied to each other, being so curved, give a greater extent of apposition than if they were plane, and, at the same time that they allow of a limited yielding of the sacrum to pressure, keep the surfaces continually in contact. And we shtill find that, although the general shape of the articular surface is rendered angular by the "joggle " b (fig. 88. A), the groove and corresponding iliac ridge form a regular cres centic curve, or segment of a circle d, f, e, of which, in fact, the central internal projec tion of the tuberosity of the ilium above at a is the centre. Now it is to this prominence that the powerful deep posterior and interos seous sacro-iliac lig,aments are mainly fixed above; and it is by being suspended by and moving on them in the radius a, f, that the sacrum slides on the ilia dovvnwards and back wards in the direction of this groove on the reception of force from above.
That this motion, though limited, does take place, and in this direction, may very readily be proved, on the detached pelvis, by striking directly downwards on the upper extremity of three or four lumbar vertebrm cut off with it. The impulse will be almost entirely felt at the tip of the coccyx, in a direction upwards and backwards. That portion of it which is directed immediately downwards is checked by the powerful liga ments above mentioned, and is but little felt at the sacral promontory.
If a section of the whole pelvis, in tbe direction of the cotylo-sacral arch is made, as in the next figure, a very important element in the mechanism of the sacro-iliac articula tion is brought to view ; viz., the deep pos terior and interosseous sacro-iliac ligaments, (d, e.) These ligaments are continuous one into the other, becoming shorter downwards, as the distance between the bones becomes less. They narrow also antero-posteriorly, so as finally to be received into the retiring angle formed by the limbs of the articular facet, at which point they are seen in the transverse section in the sacral axis in fig. 88. B, f. They are attached, externally, to the central backwards, and are curved also a little wards, the better to resist inward traction, prominence on the inner surface of the iliac tuberosities, a, a', which project upwards and following the lesser curve of the iliac crest. This thickened central portion of the tubero sities is placed above the angle of the articular facet, in the line of direction of the cotylo sacral arch produced upwards through it. In the accompanying figure, the section, made al most in the plane of the pelvic brim, cuts di rectly through it. Passing downwards and in wards, the powerful fibres of these ligaments are attached to the upper external part of the posterior surface of the sacrum, b ; and they suspend the sacrum between them some what in the manner of a suspension bridge, of which the iliac tuberosities are the sus pending buttresses. This arrangement evi dently considerably adds to the yielding elas ticity of the sacro-iliac joint, and does much to lessen the concussions passing through it. It is evident also that it is in these ligaments that the most powerful preven tive to anterior and downward displacement of the sacrum resides ; for this could not take place without absolute rupture of their numerous„fibres, resisting, as they do, all motion of the sacrum, except, in the limited sweep of the radii they form, a motion which exactly coincides with the movement of the crum proved by the experiment just mentioned.