Now we'll look at the muscles. Most movements of the vertebral column are produced by an extensive set of muscles, that run all the way along the back of the spine. They’re known collectively as the paravertebral muscles.
The highest of them are attached to the base of the skull, the lowest ones arise from the sacrum and iliac crest, some in between are attached to the backs of the ribs, and many are attached to the transverse and spinous processes of the vertebrae. We’ll build up our picture of these muscles from the inside to the outside.
This is a dissection of the mid-thoracic region. On the left side, all the paravertebral muscles are in place, partly hidden beneath a covering layer of fascia. On the right side, all the paravertebral muscles have been removed. We’re not concerned at present with these outlying muscles, the levators, and the intercostals.
Now we’ll add the paravertebral muscles to the picture, starting with the ones that lie deepest, the short and long rotator muscles.
Each short rotator goes from a transverse process, to the base of the spinous process of the vertebra above. Each long rotator goes to the same point, on the next vertebra but one.
The rotators are overlaid by this series of more obliquely running strips of muscle which together form one long muscle, the multifidus muscle. Each segment of the multifidus arises from a transverse process, and inserts on the sides of the spinous processes two to four vertebrae above.
The rotators and the multifidus extend the whole length of the spine. Their action is to produce rotation of the upper part of the spine, towards the opposite side
These deep, rotating muscles are overlaid by much larger muscles. To get a picture of these remaining paravertebral muscles, we’ll divide them into a lower group, the long muscles of the lumbar and thoracic regions, and an upper group, the long muscles of the back of the neck. The two groups overlap. We’ll look at the lower group first. They’re known collectively as the erector spinae muscles. They form a large mass of muscle, extending all the way from the sacrum, to the upper part of the thorax.
At their origins, they’re joined together. Passing upward, they separate out into three somewhat distinct muscles, the spinalis, the longissimus thoracis, and the iliocostalis lumborum.
The erector spinae muscles arise from this massive common tendon of origin, which is attached to the spinous processes of the lumbar vertebrae, to the back of the sacrum, and to the iliac crest.
Spinalis inserts onto the spinous processes of the upper thoracic vertebrae. Longissimus thoracis inserts on the lower nine ribs, and the adjoining transverse processes. Iliocostalis lumborum inserts further out, on the lower six ribs.
The erector spinae muscles are important in keeping the body upright. The action that they have depends on whether they contract on both sides at once, or on one side only. When they contract on one side only, they produce lateral flexion of the spine, to one side, or the other.
When they contract on both sides at once, their action produces extension of the lumbar and thoracic spine, straightening our back as we stand up from a stooping position, and keeping it straight as we lean forward.
The action of the erector spinae group is counteracted by muscles of the abdominal wall, which we’ll see later in this tape.
Above the erector spinae muscles, and overlapping with them, are the long muscles of the back of the neck, which we’ll look at just briefly at this point. They’re the obliquely running splenius and longissimus muscles, and beneath them the vertically running semispinalis - here’s its upper end.
We’ll look at the muscles of the neck in a lot more detail, in volume 4 of this atlas.