The sacrum (os sacrum) is a large, triangular bone at the base of the vertebral column and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part or base articulates with the last lumbar vertebra, its apex with the coccyx. It is curved upon itself and placed very obliquely, its base projecting forward and forming the prominent sacrovertebral angle when articulated with the last lumbar vertebra; its central part is projected backward, so as to give increased capacity to the pelvic cavity. It is concave facing forwards, thus its curvature is considered a kyphosis.
The pelvic surface (facies pelvina) is concave from above downward, and slightly so from side to side. Its middle part is crossed by four transverse ridges, the positions of which correspond with the original planes of separation between the five segments of the bone. The portions of bone intervening between the ridges are the bodies of the sacral vertebrae. The body of the first segment is of large size, and in form resembles that of a lumbar vertebra; the succeeding ones diminish from above downward, are flattened from before backward, and curved so as to accommodate themselves to the form of the sacrum, being concave in front, convex behind. At the ends of the ridges are seen the anterior sacral foramina, four in number on either side, somewhat rounded in form, diminishing in size from above downward, and directed lateralward and forward; they give exit to the anterior divisions of the sacral nerves and entrance to the lateral sacral arteries. Lateral to these foramina are the lateral parts of the sacrum, each consisting of five separate segments at an early period of life; in the adult, these are blended with the bodies and with each other. Each lateral part is traversed by four broad, shallow grooves, which lodge the anterior divisions of the sacral nerves, and are separated by prominent ridges of bone which give origin to the Piriformis muscle.
If a sagittal section be made through the center of the sacrum, the bodies are seen to be united at their circumferences by bone, wide intervals being left centrally, which, in the fresh state, are filled by the intervertebral fibrocartilages. In some bones this union is more complete between the lower than the upper segments.
The dorsal surface (facies dorsalis) is convex and narrower than the pelvic. In the middle line it displays a crest, the middle sacral crest, surmounted by three or four tubercles, the rudimentary spinous processes of the upper three or four sacral vertebrae. On either side of the middle sacral crest is a shallow groove, the sacral groove, which gives origin to the Multifidus, the floor of the groove being formed by the united laminae of the corresponding vertebrae. The laminae of the fifth sacral vertebra, and sometimes those of the fourth, fail to meet behind, and thus a hiatus or deficiency occurs in the posterior wall of the sacral canal. On the lateral aspect of the sacral groove is a linear series of tubercles produced by the fusion of the articular processes which together form the indistinct sacral articular crests. The articular processes of the first sacral vertebra are large and oval in shape; their facets are concave from side to side, look backward and medialward, and articulate with the facets on the inferior processes of the fifth lumbar vertebra. The tubercles which represent the inferior articular processes of the fifth sacral vertebra are prolonged downward as roundedprocesses, which are named the sacral cornua, and are connected to the cornua of the coccyx. Lateral to the articular processes are the four posterior sacral foramina; they are smaller in size and less regular in form than the anterior, and transmit the posterior divisions of the sacral nerves. On the lateral side of the posterior sacral foramina is a series of tubercles, which represent the transverse processes of the sacral vertebrae, and form the lateral crests of the sacrum. The transverse tubercles of the first sacral vertebra are large and very distinct; they, together with the transverse tubercles of the second vertebra, give attachment to the horizontal parts of the posterior sacroiliac ligaments; those of the third vertebra give attachment to the oblique fasciculi of the posterior sacroiliac ligaments; and those of the fourth and fifth to the sacrotuberous ligaments.
The lateral surface is broad above, but narrowed into a thin edge below. The upper half presents in front an ear-shaped surface, the auricular surface, covered with cartilage in the fresh state, for articulation with the ilium. Behind it is a rough surface, the sacral tuberosity, on which are three deep and uneven impressions, for the attachment of the posterior sacroiliac ligament. The lower half is thin, and ends in a projection called the inferior lateral angle; medial to this angle is a notch, which is converted into a foramen by the transverse process of the first piece of the coccyx, and transmits the anterior division of the fifth sacral nerve. The thin lower half of the lateral surface gives attachment to the sacrotuberous and sacrospinous ligaments, to some fibers of the Gluteus maximus behind, and to the Coccygeus in front.
The base of the sacrum (basis oss. sacri), which is broad and expanded, is directed upward and forward. In the middle is a large oval articular surface, the upper surface of the body of the first sacral vertebra, which is connected with the under surface of the body of the last lumbar vertebra by an intervertebral fibrocartilage. Behind this is the large triangular orifice of the sacral canal, which is completed by the laminae and spinous process of the first sacral vertebra. The superior articular processes project from it on either side; they are oval, concave, directed backward and medialward, like the superior articular processes of a lumbar vertebra. They are attached to the body of the first sacral vertebra and to the alae by short thick pedicles; on the upper surface of each pedicle is a vertebral notch, which forms the lower part of the foramen between the last lumbar and first sacral vertebrae. On either side of the body is a large triangular surface, which supports the Psoas major and the lumbosacral trunk, and in the articulated pelvis is continuous with the iliac fossa. This is called the ala; it is slightly concave from side to side, convex from before backward, and gives attachment to a few of the fibers of the Iliacus. The posterior fourth of the ala represents the transverse process, and its anterior three-fourths the costal process of the first sacral segment.
The apex (apex oss. sacri) is directed downward, and presents an oval facet for articulation with the coccyx.
The vertebral canal (canalis sacralis; sacral canal) runs throughout the greater part of the bone; above, it is triangular in form; below, its posterior wall is incomplete, from the non-development of the laminae and spinous processes. It lodges the sacral nerves, and its walls are perforated by the anterior and posterior sacral foramina through which these nerves pass out.
The sacrum articulates with four bones; the last lumbar vertebra above, the coccyx below, and the hip bone on either side.
The sacrum is noticably sexually dimorphic (differently-shaped in males and females). In the female the sacrum is shorter and wider than in the male; the lower half forms a greater angle with the upper; the upper half is nearly straight, the lower half presenting the greatest amount of curvature. The bone is also directed more obliquely backward; this increases the size of the pelvic cavity and renders the sacrovertebral angle more prominent. In the male the curvature is more evenly distributed over the whole length of the bone, and is altogether greater than in the female.
The sacrum, in some cases, consists of six pieces; occasionally the number is reduced to four. The bodies of the first and second vertebrae may fail to unite. Sometimes the uppermost transverse tubercles are not joined to the rest of the ala on one or both sides, or the sacral canal may be open throughout a considerable part of its length, in consequence of the imperfect development of the laminae and spinous processes. The sacrum, also, varies considerably with respect to its degree of curvature.
Most of this copied and pasted from the 1917 Gray's anatomy, which is in the public domain: The Sacral and Coccygeal Vertebrae