Final answer:
The cuticle is the scale-like, outermost layer of the hair shaft made up of overlapped hard, keratinized cells that protect the inner layers of hair.
Step-by-step explanation:
The scale structure covering the exterior of the hair is known as the cuticle. It is the outermost layer of the hair shaft and is composed of very hard, keratinized cells. These cells originate from the hair matrix and overlap like shingles on a roof, providing a protective layer for the underlying cortex, which is the middle zone of the hair shaft responsible for the hair's mechanical properties and color. The innermost zone called the medulla may or may not be present and is surrounded by the cortex. This protective cuticle helps prevent damage to the hair shaft and maintains the overall health of the hair.