Dorsal surface of sacrum
Encyclopedia
The dorsal surface of sacrum
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
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine 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 connects with the last lumbar vertebra, and bottom part with the coccyx...
is convex and narrower than the pelvic
Pelvic surface of sacrum
The pelvic surface of sacrum 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...
.
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
Lamina of the vertebral arch
The laminæ are two broad plates, extending dorsally and medially from the pedicles, fusing to complete the roof of the vertebral arch.Their upper borders and the lower parts of their anterior surfaces are rough for the attachment of the ligamenta flava....
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
Sacral canal
The vertebral canal runs throughout the greater part of the sacral bone; above, it is triangular in form; below, its posterior wall is incomplete, from the non-development of the laminæ and spinous processes....
.
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
Articular processes
The articular processes or zygapophyses of a vertebra, two superior and two inferior, spring from the junctions of the pedicles and laminæ...
of the first sacral vertebra are large and oval in shape; their facets
Zygapophysial joint
A zygapophysial joint is a synovial joint between the superior articular process of one vertebra and the inferior articular process of the vertebra directly above it...
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
Coccyx
The coccyx , commonly referred to as the tailbone, is the final segment of the vertebral column. Comprising three to five separate or fused vertebrae below the sacrum, it is attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement between...
.
Lateral to the articular processes are the four posterior sacral foramina
Posterior sacral foramina
Lateral to the articular processes of the sacrum 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....
; they are smaller in size and less regular in form than the anterior
Anterior sacral foramina
At the ends of the transverse ridges of the pelvic surface of the sacrum 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...
, and transmit the posterior divisions of the sacral nerves
Sacral nerves
The five sacral nerves emerge from the sacrum. Although the vertebral components of the sacrum are fused into a single bone, the sacral vertebrae are still used to number 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
Lateral sacral crest
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 sacral crest....
.
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.