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That part of the vertebral column which is directly connected with, or forms a part of, the pelvis.

That part of the vertebral column which is directly connected with, or forms a part of, the pelvis.

The lowest part of the backbone.

Small bone separating final lumbar vertebra and coccyx.

The os sacrum or bone forming the termination of the vertebral column, usually consisting of several fused vertebrae.

because it contributes to protect the genital organs, which were considered sacred, or because it was offered in sacrifice. The bone, which forms the posterior part of the pelvis, and is a continuation of the vertebral column. The sacrum is symmetrical and triangular, situate at the posterior part of the pelvis, and concave anteriorly. It has,-1. A pelvic or anterior surface, which corresponds to the pelvic cavity, and has four transverse furrows, that point out the parts where the primitive portions of the bone were separated from each other. On each side there are four foramina, called anterior sacral; these are oblique, and decrease in size, from above downwards. Through them pass the anterior branches of the sacral nerves. 2. A spinal or posterior surface, which is convex, rough, and covered by muscles. Upon the median line are four horizontal, tubercular eminences, forming a continuation of the spinous processes of the vertebrae; and below, a triangular notch, where the sacral canal terminates. On each side of the surface are four foramina, called posterior sacral, which are smaller than the anterior, and give passage to the posterior branches of the sacral nerves. 3. The vertebral surface or base. Its greatest extent is transversely. It is articulated with the last lumbar vertebra. On it is seen the orifice of the sacral canal, of a triangular shape, which decreases in width from above downward; terminates the vertebral canal; is lined by a prolongation of the cerebral membranes; contains the fasciculi of the sacral nerves; communicates, externally, by the sacral foramina; and terminates at the triangular notch on the posterior surface of the bone. At the sides of the upper orifice of the sacral canal are two articular processes, which receive the articular processes of the last lumbar vertebra. The projection, formed by the union of the base of the sacrum with the last lumbar vertebra, is called the Promontory, Promontorium Ossis Sacri, Great Angle of the Sacrum or Sacro-vertebral Angle. 4. A coccygeal face or apex, (F.) Face coccygienne ou Sommet, which is very narrow, and is articulated with the coccyx. 5. Two lateral margins, which have, above, a broad, oblique surface, articulated with the ilium. The sacrum is developed by 30 to 36 points of ossification.

The triangular bone between and behind the two ilia.

The sacred bone (so called from an ancient belief that it was indestructible); a bone of the back part of the pelvis (really made up of five vertebrae) situated between the ossa innominata and articulating with them, forming the "keystone" of the pelvic arch. [Lat.]