The sacrum, much the largest piece of the vertebral column, is placed, when the body is in the erect position, at the superior and posterior part of the pelvis, beneath the last lumbar vertebra, above the coccyx, and between the ossa innominata, between which it is inserted, in some measure like a keystone into an arch. 

The sacrum is placed very obliquely. It projects backwards from the upper margin, receding to give capacity to the pelvis, and it therefore forms, with the body of the last lumbar vertebra, a projection named the sacro-vertebral angle, or promontory. Its figure is triangular in its general outline, — the base being upwards; concave anteriorly, convex posteriorly. We consider successively its surfaces, borders, and extremities. 

The anterior or pelvic surface, which is shown in the figure, is concave from above downwards, slightly so from side to side, and marked by four transverse lines, 1 indicating its original division into five pieces ; laterally it presents four foramina, 3 (anterior sacral,) for the transmission of the anterior branches of the sacral nerves. These are directed outwards into grooves which lead from them, and diminish gradually in size from above downwards; external to the foramina the surface gives attachment to the pyramidalis muscle. 

The posterior or spinal surface is narrower than the anterior, for the bone is somewhat wedge-shaped from before backwards as well as from above down. This surface is convex, and presents along the median line four small eminences, usually connected so as to form a ridge; these are rudiments of the spinous processes; and beneath them is a triangular groove, or rather an opening, marking the termination of the sacral canal. The margins of the opening present two tubercles, which give attachment to the ligament that closes in the canal, and the inferior pair (sacral cornua) articulate with the horns of the coccyx.

At each side of the median line are two sets of tubercles and between these is the groove, pierced by the posterior sacral foramina, which are much smaller than the anterior,and transmit the posterior branches of the sacral nerves. The groove represents that situated over the plates of the vertebrae above the sacrum, and one row of the tubercles corresponds to the lumbar articular tubercles, the other ranges with transverse processes. 

Each pair of foramina (anterior and posterior) lead from a single foramen situated within the bone, and this is analogous to the inter-vertebral foramen in other parts of the column. 

The borders, or lateral surfaces of the sacrum, present two distinct parts, — one superior, the other inferior. The superior (iliac) is large and irregular, 4 and in front is, in the fresh state, covered with cartilage, and articulated with the ilium ; whilst posteriorly it is concave and rough for the attachment of strong ligaments. The anterior cartilaginous part is often named " the auricular surface:'' The inferior part of the lateral surface 6 is thin and sinuous, and gives attachment to the sacro-sciatic ligaments. A small indentation 11 terminates this border, which, with the corresponding extremity of the coccyx, forms a notch for the transmission of the fifth sacral nerve. 

The base, or superior extremity, broad, and expanded, presents, towards the middle line, an oval surface, 6 cut off obliquely, which articulates with the likewise oblique body of the last lumbar vertebra; behind this a triangular aperture marking the orifice of the sacral canal ; on each side a smooth convex surface, inclined forwards, and continuous with the iliac fossa ; an articular process, 8 concave from side to side, which looks backwards and inwards, and receives the inferior articular process of the last lumbar vertebra. Before each articular process is a groove, forming part of the last lumbar inter-vertebral foramen, and behind them is a curved, sharp, and depressed border which bounds the sacral canal, and therefore corresponds with the laminae of the vertebras, and gives attachment to the last ligamentum subflavum. 

The apex, or inferior extremity, directed downwards and forwards, presents an oval convex surface, which articulates with the coccyx. 

The sacrum in its interior contains much loose spongy substance, and its exterior layer is but moderately compact. Its central part is also hollowed into a canal (sacral), which curves from above downwards as the bone does; it is of a triangular form, and gradually narrows as it descends. The canal ends on the posterior surface of the bone between the sacral cornua. 

Attachments of muscles

The sacrum gives attachment, by the lateral parts of its anterior surface, to the piriformis muscles ; by its posterior surface at each side to the gluteus maximus, sacro-lumbalis, longissimus dorsi, and multifidus spinas; by the inferior part of each border to part of the coccygeus ; and by the outer parts of its base to the iliacus muscle of each side. 

Articulations

The sacrum articulates with the last lumbar vertebra, the two iliac bones, and the coccyx. 

Peculiarities of the sacrum

The peculiarities of the sacrum are very numerous. 

• 1. In some cases this bone consists of six instead of five pieces, and it has been  found — but much more rarely — reduced to four.
• 2. Occasionally the bodies of the first and second sacral vertebras are not joined, although complete union has taken place in every other part. 
• 3. The lower end of the sacral canal may be open for some extent, in consequence of the vertebral laminae not having grown together, [and occasionally it remains open the whole length of the sacrum.
• 4. In no respect does the sacrum vary more in different skeletons than in the degree of its curve. It is difficult to submit the peculiarities in this respect to a precise and sufficiently comprehensive arrangement; still, after examining a considerable number of skeletons, the majority appeared to admit of being grouped into three sets, as follows :
  • a. In one series the anterior surface was comparatively straight, and the slight bend which existed was situated near the lower end. 
  • b. Another group contrasted strongly with the preceding, — the bone being much curved in its whole length, but especially about its middle.
  • c. A considerable number may be described as holding an intermediate place between the two foregoing classes. The degree of curve was moderate, and chiefly affected the lower third of the bone. 

Differences in the sexes

Besides possessing the ordinary distinctive character of all parts of the skeleton, — viz., more regularity and smoothness of surface, — the sacrum of the female body is, proportionally to the size of the pelvis or of the skeleton, broader than that of the male. 

The degree in which the bone is bent has been relied on by anatomical writers to distinguish between the sexes; but, on comparing their statements, it will be found that they are contradictory — some assigning the greater curve to the female, others, on the contrary, to the male. The measurement of a considerable number of those bones taken from both sexes, has shown me that the curvature cannot be relied on as a distinctive character. I find that the general remarks made in the preceding paragraphs on the varieties presented by the sacrum, with reference to the point in question, are applicable either to the female or the male taken singly, with only this reservation, that those bones which were most curved, and which constitute the second series in the classification there ventured on, commonly belong to the male body. 

It is said by many good observers, that the sacrum usually inclines backwards from the direction of the lumbar vertebras to a greater extent in the skeleton of the female than of the male, — thus retiring more from the cavity of the pelvis, and forming a more prominent sacro-vertebral angle.