Final answer:
The external part of intervertebral discs is the anulus fibrosus, a tough, fibrous outer layer composed of fibrous cartilage that provides attachment and flexibility between vertebrae. It can lead to a herniated disc if damaged.
Step-by-step explanation:
The external part of intervertebral discs is called the anulus fibrosus. This portion of the disc is a tough, fibrous outer layer that encircles the softer, inner part known as the nucleus pulposus. The anulus fibrosus is composed of fibrous cartilage, and it has a ring-like arrangement that is firmly anchored to the margins of adjacent vertebral bodies. It is designed to provide both strong attachments between vertebrae and flexibility, allowing the spine to move. As individuals age, the water content in the nucleus pulposus decreases which can lead to a reduction in disc thickness and elasticity. If the anulus fibrosus is damaged, it may result in conditions such as a herniated disc.