Final answer:
The thickened stratum corneum over which the free edge of the nail extends is called the hyponychium. It acts as a protective barrier and helps to anchor the nail body in place.
Step-by-step explanation:
The thickened stratum corneum over which the free edge of the nail extends is called the hyponychium. The hyponychium is a thickened layer of stratum corneum that lies below the free edge of the nail. It acts as a protective barrier and helps to anchor the nail body in place.