As a whole the spinal column is a series of stacked vertebrae which attach to the pelvis, ribs and skull. While each of the vertebrae has protrusions that act as points of attachment and as levers or muscles to more effectively act on them, the pelvis, ribcage and skull could be considered as extra long levers.
And so it can help to think of the ribs pelvis and head as, on occasion, extensions of the spine.
The part of the spine that joints the two halves of the pelvis together is called the sacrum. It looks like a downward pointing arrow with the tip (called the tailbone or coccyx) just behind the anus. The connections between the sacrum and the two hip bones are called the SI joints.
The SI joints and the joint where the two halves join at the front of the pevis allow the pelvis some flexibility in shape. These joints allow the hip bones and sacrum to move relative to each other.
An important part of spinal anatomy can be understanding how the muscles of the pelvic floor, abdomen and hip joint all can affect the shape of the pelvis via the SI Joints.
The lumbar spine connects the pelvis to the ribcage and is made up of five vertebrae. These vertebrae are quite large and contrary to the usual notion that they are large to support the weight of the upper torso, I'd suggest that they are large so as to be able to transfer forces between ribcage and pelvis and vice versa.
The thoracic spine is the part of the spine to which the ribs attach and it has twelve vertebrae. I like to think in terms of a lower and upper thoracic spine. The lower part is the part whose corresponding ribs form the arch at the front of the ribcage. Like the lumbar spine this part has five vertebrae. The upper part has ribs that attach directly to the sternum. This part, like the cervical spine, has seven vertebrae.
The ribs can be divided into three categories, true, false and floating. The true ribs are the upper seven pairs of ribs that attach directly to the sternum. The false ribs attach to the sternum via cartelide. The floating ribs are the two lower most pair of ribs whose ends are free.
The ribcage, like the pelvis, is a flexible structure. Muscles that attach to the thoracic spine, and also between ribs can help change the shape of the ribcage.
One important reason for the sternum is that it makes the upper ribcage more stable for the arms. As a comparison, some types of turtles developed shells in particular to enable them to dig more efficiently. The shell provides a foundation for stronger digging actions.
In a similar way, if you look at an ostrich or emu skeleton, they have large sternums so that the muscles that work on the wings have a strong foundation. Similarly, we have a sternum so that we can use our arms effectively and so that we can use them to transmit or deal with forces.
Interestingly the shoulder blade tends to float in the region of the upper part of the ribcage, the part that corresponds to the sternum.
The cervical spine joins the ribcage to the head and has seven vertebrae.
As a whole the spinal column has two main functions. One is that it acts as a bony support and connecting device for the head, ribs and pelvis. It also serves as an information channel carrying nerves from the brain to the rest of the body.
The main supporting function is carried out by the bodies of the vertebrae which are connected to each other via intevertebral disks. These disks create space between adjacent vertebrae while at the same time keeping them connected and allowing them to move realtive to each other.
To the rear of the bodies a channel is formed for the passage of nerves. To the rear of each vertebrae is a spinous process which point rearwards and two tranvserves processes which point outwards and backwards to either side.
The spinous processes can limit backward bending of the vertebrae relative to each other.
All of the processes serve as points of muscular attachment and so can be used to exert leverage on each vertebrae.
Each vertebrae also has upper and lower joint facets which both limit and define the movement possibilities of the vertebrae relative to each other.
Passages exist between adjacent vertebrae so that nerves can leave the spine channel and venture to other parts of the body.
Various levels and layers of muscles enable the spine to move in different ways.
Working from smaller to larger, the muscles of the spinal column span can span a single vertebral joint or many. Muscles that joint adjacent transverse processes and adjacent spinous processes can help in side bending that spinal segment or backbending it.
Muscles that jump from transverse process to spinous process can also be used to backbend that spinal segment but in additon can twist it.
Larger muscle groups work similiarly but work across two or more spinal segments.
The spinal erectors tend to be longer muscles that work across the back of the pelvis, ribcage and head as well as across the vertebrae. All together these muscles allow the spine a lot of flexibility in bending in different ways.
Because the spine attaches to the pelvis, ribcage and head, muscles that work between these structures can also affect the shape and stabilty of the spine.