The nail plate grows forward on the nail bed upon a series of 'rails and grooves' much like a train rides forward on its tracks. The rails are on the surface of the nail bed and the grooves are on the underside of the nail plate. Sometimes, one of the rails will become enlarged and can actually push itw way up and through the nail plate, causing it to split into two sections. When the nail plate becomes dried out due to age or repeated and prolonged use of strong solvents, the rails and grooves become apparent on the nail plate surface as 'ridging'. You can try rehydrating the nail plate with a good quality nail and cuticle oil that contains Jojoba and Vitamin E oils.