Now that we’ve looked at the dry bones of the vertebral column, let’s look at the structures that hold the bones together, and that enable them to move. We’ll look first at the intervertebral disks, then at the ligaments of the vertebral column, then at the posterior joints.
These structures are arranged in a similar way from the top of the spine to the bottom. We’ll be looking at all of them in the lumbar region.
Here’s an intervertebral disk. The disk is a massive pad of fibrocartilage, that’s firmly attached to the vertebral body above and below, all the way round the circumference.
If we cut through a disk and look at it from above, we see that it’s made of concentric layers of material. The disk consists of an outer ring of tough fibrocartilage, called the anulus fibrosus, and a soft center of almost liquid material, called the nucleus pulposus.
The disk is solid enough to transmit the weight of the body, and it's flexible enough to permit movement between the vertebrae. The side of the intervertebral disk forms the anterior margin of the intervertebral foramen, through which the spinal nerve emerges.
The vertebrae are also held together by ligaments. Some of these go from vertebra to vertebra; some run the length of the spine. Starting at the back, we’ll look at the ligaments which hold the spinous processes together, the interspinous and supraspinous ligaments; then we'll look at the ligament that holds the laminae together, the ligamentum flavum. Then we’ll look at the two ligaments that help to hold the bodies together: the anterior and posterior longitudinal ligaments.
First, the interspinous ligaments - here they are. They run from the lower edge of one spinous process to the upper edge of the next one. Now we’ll add the supra-spinous ligament to the picture.
The supra-spinous ligament merges with the interspinous ligaments. It runs the whole length of the vertebral column, connecting the tips of the spinous processes. The supraspinous ligament serves as a midline attachment for some important muscles, as we’ll see later. These ligaments help to limit flexion of the spine.
The structure, or structures, that chiefly limit flexion of the vertebral column is the series of short ligaments that hold the laminae together, which are known collectively as the ligamentum flavum.
The ligamentum flavum lies on the front of the laminae. To see it, we’ll cut though the pedicles of all the vertebrae, along this line, and look at the laminae from the inside.
Here’s the ligamentum flavum. It goes from one lamina to the next all the way down the spine. Here, where it’s been cut through we can see how thick it is. The ligamentum flavum is made of yellowish fibro-elastic tissue, hence its name, which means yellow ligament.
Next we’ll look at the two ligaments which hold the vertebral bodies together - the anterior and posterior longitudinal ligaments. The anterior is the stronger of the two - here it is. The anterior longitudinal ligament covers the front and sides of the vertebral bodies. It runs the whole length of the vertebral column. We’ll cut through it along this line to see it better.
The anterior longitudinal ligament is thick and strong. It’s attached to the upper and lower edges of each vertebral body. It limits extension of the spine. In extension, the tightness of the anterior longitudinal ligament helps to prevent backward and forward movement of the vertebral bodies relative to each other.
The posterior longitudinal ligament runs along the back of the vertebral bodies. To see it we’ll divide the pedicles along this line again, and look at the bodies by themselves.
Here’s the posterior longitudinal ligament. It’s narrow where it overlies each body, and it widens out to cover the back of each disk. The posterior longitudinal ligament helps in a small way to limit flexion of the vertebral column.