Final answer:
The flat, round structure between two vertebrae is the intervertebral disc, which cushions and allows for movement of the spine.
Step-by-step explanation:
The flat, round plate-like structure between two vertebrae is called an intervertebral disc. This disc is composed of a tough, fibrous outer layer known as the anulus fibrosus and a soft, gel-like center called the nucleus pulposus. It plays several crucial roles, such as providing padding to absorb shocks during activities like walking and running, allowing for flexibility and movement of the spinal column, and serving as a ligament to bind the vertebrae together. The thickness of the intervertebral discs can vary, with thinner discs in the cervical region and thicker ones in the lumbar region, due to the different amounts of body weight they support. Additionally, as people age, the nucleus pulposus tends to harden and lose its elasticity, affecting the disc's capacity to absorb shock.