The intervertebral disc is found between the vertebral bodies. It is a complex structure that supports the weight of the body and, with the facet joints, permits a significant range of motion. The disc is made up of fibrocartilage and has two parts: the nucleus pulposus and the annulus fibrosus. The nucleus pulposus is a gel-like material located in the center of the disc. It has a high water content, which allows it to act as a cushion and distribute loads onto the vertebral body end plates and to the annulus. The water content of the nucleus decreases with age, which can contribute to some of the conditions discussed in later chapters. The disc has no blood supply of its own. The annulus fibrosus is the outer portion of the disc. The annulus consists of 15 to 25 layers of collagen, much like the layers of a truck tire. This structural design allows the annulus to contain the nucleus under pressure, and to help hold the vertebral bodies in place. The annulus is weakest at the back, on either side of the midline, which is often the location of disc herniations.

The bony end plate covers the top and bottom surfaces of the disc, and is the interface between the nucleus and the vertebral body. The posterior longitudinal ligament connects all the vertebral bodies along the back and lines the spinal canal. It is a fibrous band that extends from the base of the skull to the sacrum. The anterior longitudinal ligament is a broad band on the front surfaces of the vertebral bodies. The supraspinous ligament runs across the top of the spinous processes. The paired ligament that lies between adjacent lamina, and protects the neural elements beneath it, is the ligamentum flavum, sometimes referred to as the “yellow ligament.” It is made up of elastic fibers, and there is an opening in the middle between the two paired halves. The intertransverse ligaments connect adjacent transverse processes. The interspinous ligament connects adjacent spinous processes. The ligamentum nuchæ is a fibrous membrane in the neck. It extends from the external occipital protuberance and median nuchal line to the spinous process of the bottom cervical vertebra.