. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 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
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 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 displa
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