144 



PELVIS. 



prominence on the inner surface of the iliac backwards, and are curved also a little out- 

 tuberosities, X, which project upwards and wards, the better to resist inward traction, 



fig. 89. 



ScctioJi of the pelvis and heads of the thigh bones, made in the direction of the cotylo-sacral arch, a little lielow 

 the pelvic brim ; showing the antero-posterior sacral wedge, the suspending office and oblique direction 

 of the posterior sacro- iliac ligaments, and the wavy section of the joint, a, iliac tuberosities ; b, c, 

 antero-posterior sacral wedge; d, deep posterior sacro-iliac ligaments; e, interosseous ligaments; 

 y, auricular groove; c, sacral joggle; g, c, cotylo- sacral rib. (Drawing made from a recent section.) 



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 sectioti, 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 mover entof the sa- 

 crum proved by the experiment just mentioned. 

 But these ligaments, from their oblique 

 direction inwards, at the same t'me 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, overcome 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 " exlrados " at the 

 apex, a, and to be brought within the inner 

 surface or " 'mil-ados'" of the arch, b, on each 

 side ; and this causes the voussoirs, a d, d r, 

 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, 



