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3.1.3 Lumbar vertebrae, sacrum

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(3.35)

Now we’ll take a look at a lumbar vertebra. The body is massive. The transverse processes are small, the spinous process is broad, and points almost straight backwards.

The upper articular processes of lumbar vertebra face inward, the lower ones face outward. Because of this arrangement, there’s almost no rotation between lumbar vertebrae. The movements that can occur in the lumbar spine are flexion, and extension, and lateral flexion to either side.

Lastly, we’ll look at the sacrum. Besides being the lowest part of the spine, the sacrum is also an important part of the pelvis.

Here’s the sacrum, together with the coccyx. The sacrum is formed by five vertebrae fused together. From top to bottom it has a marked backward curve. When we’re standing upright, the sacrum is oriented just as we see it here. The upper part of this backward-facing dorsal surface is angled at about 45° to the vertical. The upper part of this forward-facing pelvic surface is more nearly horizontal than vertical. On the dorsal surface there are two articular processes, for the fifth lumbar vertebra.

The lowest intervertebral disk is quite wedge-shaped. Its shape accounts in part for the very marked curvature of the spine between the fourth lumbar vertebra and the sacrum. The most anterior point on the sacrum is called the sacral promontory. The vertebral canal continues down the back of the sacrum.

From within the vertebral canal, the anterior rami of the spinal nerves S1 to S4 emerge form these pelvic sacral foramina. The posterior rami emerge from these dorsal sacral foramina. The vertebral canal ends at this opening, the sacral hiatus, that’s shaped like an upside down V.

This curved auricular surface articulates on each side with the upper part of the innominate bone, or hip bone, to form the ...

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(3.35)

Now we’ll take a look at a lumbar vertebra. The body is massive. The transverse processes are small, the spinous process is broad, and points almost straight backwards.

The upper articular processes of lumbar vertebra face inward, the lower ones face outward. Because of this arrangement, there’s almost no rotation between lumbar vertebrae. The movements that can occur in the lumbar spine are flexion, and extension, and lateral flexion to either side.

Lastly, we’ll look at the sacrum. Besides being the lowest part of the spine, the sacrum is also an important part of the pelvis.

Here’s the sacrum, together with the coccyx. The sacrum is formed by five vertebrae fused together. From top to bottom it has a marked backward curve. When we’re standing upright, the sacrum is oriented just as we see it here. The upper part of this backward-facing dorsal surface is angled at about 45° to the vertical. The upper part of this forward-facing pelvic surface is more nearly horizontal than vertical. On the dorsal surface there are two articular processes, for the fifth lumbar vertebra.

The lowest intervertebral disk is quite wedge-shaped. Its shape accounts in part for the very marked curvature of the spine between the fourth lumbar vertebra and the sacrum. The most anterior point on the sacrum is called the sacral promontory. The vertebral canal continues down the back of the sacrum.

From within the vertebral canal, the anterior rami of the spinal nerves S1 to S4 emerge form these pelvic sacral foramina. The posterior rami emerge from these dorsal sacral foramina. The vertebral canal ends at this opening, the sacral hiatus, that’s shaped like an upside down V.

This curved auricular surface articulates on each side with the upper part of the innominate bone, or hip bone, to form the pelvis. The joints between the sacrum and the hip bones are the sacro-iliac joints. These joints permit almost no movement.

The broad ridge on each hip bone adjoining the sacrum is the iliac crest. It’s an important muscle attachment, as we’ll see shortly.

We’ll be looking at the hip bone in more detail in the last section of this tape. For now, we’ll return to the spine.

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