PELVIS. 



145 



and the haunches rise and open above, d, d. 

 The sacro-iliac joints, being the haunches of 



d 



A, diagram of a yielding arch, a, extrados ; b, in- 

 trados ; d, d', the haunches ; c, a, c', dotted line 

 of pressure. 



B, parallelogram of forces of sacro-iliac posterior deep 

 ligaments. a, c, vertical or sustaining force ; 

 c, d, lateral or tightening force ; b, c, diagonal 

 direction of ligaments. 



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-iliac 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. B), 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 vertebras have a ten- 

 dency to throw the base or sacral promon- 

 tory downiuards, and to tilt the apex with the 

 coccyx upwards, as is seen in the experiment 

 of striking the separated extremity of the 

 lumbar vertebra; 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 B). The 

 tendency of the sacrum is to turn round the 

 axis of the sacro-iliac joints in the curve d f c, 

 (fig. 88. A.), and round the centre a (Jig. 91. 

 </). 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 sacra-sciatic ligaments (b), 

 which aid also the oblique sacra-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 vertebra? 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 



Fig. 91. 



A, vertical section of the os innominatum, with the sacrum and ligaments attached, made in the line of the 

 ischio-sacral support, a, ilio-lumbar ligament ; b, sacro-sciatic ligaments ; c, iliac tuberosity. (Drawn 

 from a recent section.) 



B, model of the mechanism of the same structures, c', d represents the yielding motion of the posterior 

 deep sacro-iliac ligaments. 



Supp. L 



