The histological divisions of the nail suggested by Lewis have been confirmed by histochemical methods. Definite differences have been demonstrated between the dorsal, intermediate, and ventral nail plates. The upper or dorsal nail plate appears to have the hardest physical structure, and this is probably related to its high calcium content rather than to its disulfide bonding. The intermediate plate is formed from a different matrix and the cells undergo less cytolysis prior to keratinization than those of the dorsal plate. This region forms the main bulk of the nail and lies between the forward-growing dorsal plate and the upward-growing ventral plate. The latter appears to be formed directly from the cells of the nail bed, and as these grow upward they are simultaneously carried forward by the ventral and dorsal portions. Diseases of the nail may primarily affect one, two, or all of the active growth centers, and it is probably because of this that the diverse morphological clinical appearances are produced.