Sacrum: Encyclopedia II - Sacrum - Surfaces

Sacrum - Surfaces

Sacrum - Pelvic surface

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.

Sacrum - Dorsal surface

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 rounded processes, 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.

Sacrum - Lateral surface

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.

Adapted from the Wikipedia article "Surfaces", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki