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Symphysis or Synchondrosis, is the union of two surfaces ;-two inclined planes, broader above than below; one formed by the sacrum, the other by the ilium. Both surfaces are incrusted by an articular cartilage. The bands which strengthen them are the two sacro-sciatic ligaments, the sacro-spinal, a sacro-iliac ligament, and some irregular fibres. The sacro-iliac ligament is a very thick assemblage of short, dense, irregular fibres, occupying the space which the sacrum and ilium leave between them, behind their articular surface. Some authors have termed some irregular fibres, situate in front of the sacro-iliac articulation, anterior sacro-iliac ligaments, in opposition to the preceding fasciae, which they call posterior sacro-iliac.