Intervertebral Disc
Fibrocartilaginous joint structure; part of the axial (core) skeleton
Also known as: Spinal disc, back disc, or vertebral cushion
Classification
Fibrocartilaginous joint structure; part of the axial (core) skeleton
Dimensions
These are generally thick, flat, circular or oval structures that sit between the main bodies of the backbones. They typically make up about one-quarter of the total height of the spinal column.
Key Features
The blue sections in the image represent the discs. They look like a rubbery sandwich or a flattened jelly donut squeezed between the hard, bony blocks of the spine. They have a tough, fibrous outer ring and a softer, more fluid center that provides a bouncing, shock-absorbing quality.
Similar Bones
Beginners might confuse discs with the bony surface of the vertebrae themselves. However, whereas the vertebrae are hard, porous, and tan or white when dry, the discs are softer tissue. In a living or fresh specimen, they have a translucent or bluish-white appearance compared to the opaque bone.
Across Species
While most mammals share a similar disc structure to provide flexibility for walking and running, specialized variations exist. In whales, these cushions are modified to handle extreme deep-sea pressure, while in jumping animals like kangaroos, they act like high-performance springs to absorb the impact of every hop. Bird necks often use a different type of saddle-shaped joint rather than true discs to allow for their extreme range of motion.
Evolutionary History
These cushions evolved as life moved from water to land. In fish, a flexible rod called a notochord provides support, but as early land-dwelling tetrapods began to deal with gravity, they needed a system that could support weight while remaining flexible. These discs developed as a way to link individual vertebrae into a cohesive, shock-absorbing pillar.
Photography Tips
To capture these clearly, take a side-view photo of the spine to show the 'stacked' arrangement. Use a light source from the side to highlight the difference in texture between the smooth, flat disc and the bumpy, irregular surface of the surrounding bone. Including a coin or ruler for scale helps show the thickness of each individual cushion.
Notes
What is the blue part?