Anatomy, Physiology, and Disorders of the Hand, Nail, and Skin
The nail unit, part by part
The nail plate is the visible body - about 100 layers of dead, hard keratin with no nerves and no blood supply, which is why trimming and filing do not hurt. It rests on the nail bed. The matrix, at the base under the proximal nail fold, is the factory that produces the plate; damage there shows up as ridges, grooves, or a permanently split nail. The lunula is the visible portion of the matrix, usually whitish and half-moon shaped. The eponychium (cuticle) is the dead tissue sealing the base; the hyponychium is the seal under the free edge; the perionychium is the skin around the nail, including the lateral folds.
Growth facts worth memorizing
Fingernails grow about 1/8 inch (3 mm) per month and are fully replaced in 4 to 6 months; toenails grow about 1/10 inch per month and take 9 to 12 months. Growth is faster in summer, in children, and on the dominant hand, and slows with age and poor circulation.
Disorders you must recognize (not treat)
Paronychia is inflamed, swollen tissue around the nail. Onychomycosis is a fungal infection that thickens and discolors the plate. Onychocryptosis is an ingrown nail. Onycholysis is separation of plate from bed. Onychorrhexis is brittleness with splitting. Onychauxis is thickening. Leuconychia is white spotting. Koilonychia is a spoon-shaped nail. Beau's lines are transverse grooves from interrupted growth. Melanonychia is a dark band.
The trap
Questions bait you into naming or treating a disease. A nail technician identifies and refers. Any redness, swelling, pain, pus, dark streak, or sudden change means no service on that nail and a referral to a physician.
Rule of thumb
Matrix equals factory: if the factory is injured, the product comes out defective.