mycontentbreak The hyponychium of the toenail is known as the quick of the nail. Hyponychium is the name given to the epithelium (a layer of thickened tissue) located underneath the nail plate, at the intersection between the spot where the skin of the tip of the toe meets the nail plate, below the free edge of the nail. The hyponychium forms a waterproof seal that helps to protect the nail bed from injury or disease.