PELVIC BONES.
The Sacrum. —This single and symmetrical bone forms the larger part of the posterior wall of the pelvis. It is a pyramid curved anteriorly, and flattened from before backwards, the base uppermost, the truncated apex below. Its base articulates with the spine, forming the promontory or sacro-vertebral angle. Below it articulates with the coccyx. It has two surfaces, two borders, a base and an apex.
Anterior Surface.—This, the pelvic face, is concave, forming the larger part of the posterior wall of the pelvic cavity. It shows several quadri lateral surfaces, separated by ridges, marking the union of the primitive portions of the bone. On either side of these lines are foramina—anterior sacral foramina—communicating with the sacral canal, and allowing the exit of the anterior sacral nerves.
Above it is 4+ inches wide, and below—where it articulates with the coccyx—it measures .9 of an inch. It is 4 inches high along a straight line, from the middle of the sacral promontory to the aacro-coccygeal articulation.
Posterior Surface.—The posterior surface is convex an I uneven. Along the median line are eminences forming the sacral crest; and, placed one above the other, they represent the spinous processes of the vertebral column.
Above, this crest is limited by a triangular opening, the orifice of the sacral canal. Below, it bifurcates into two branches embracing the lower opening of the sacral canal, and terminating in two projections called the cornua of the sacrum.
The upper opening of the sacral canal has, on each side, two vertical projections, or articular processes, for articulation with the last lumbar vertebra. On each side of the sacral crest are two shallow grooves, in which are the posterior sacral foramina for exit of the posterior sacral nerves. Outside of these grooves are the rudimentary transverse processes.
Lateral Borders.—These consist of two parts; the upper is broad, and the lower is narrow. The upper one has an articular facet shaped like a human ear, for articulation with the os innominatum. It is slightly con
cave. The lower part is rough and serves for attachment of the greater and lesser sacro-sciatic ligaments.
All authors agree that the sides of the sacrum are cut obliquely:—(1) from before backwards and from without in; —and (2) from above down wards, and from without in. This double obliquity insures solidity of union between it and the coma bone, preventing the descent of the sacrum, or any rocking motion between the two ilia.
looks upwards and forwards. It has (1) an articular sur face, very wide transversely, which joins the body of the last lumbar vertebra. (2.) Behind is the opening of the sacral canal, and the two articular projections for union with corresponding processes on the last vertebra. (3.) In front are two small concave surfaces forming grooves continuous with the first anterior sacral foramina. (4.) Further out are two prominences (rudiments of the transverse processes), surmounting a quadrilateral surface, convex from before backwards, concave from side to side, and inclining forwards. These are the ale of the sacrum.
The anterior border is blunt, forms part of the internal iliac fossa, and contributes to the formation of the superior strait.
Apex.—This is truncated, and points downwards, presenting an ellipti cal facet, convex along its trans verse diameter, which articulates with the base of the coccyx.
Sacral Canal.—The sacrum has a canal throughout its entire length. This is triangular and wide above, narrow and flat below, and forms the termination of the spinal canal. It ends in a groove lying between the sacrum and the coccyx. This groove is converted into a canal by liga ments.• It contains the caudal portion of the spinal marrow.
Coccyx.
A single and symmetrical bone, situated below the sacrum, of which it seems the prolongation.. The coccyx is shaped like a small triangular pyra mid, slightly concave. It presents an anterior and a posterior face, two lateral edges, a base and an apex.