Anatomy and Physiology of the Hands, Arms, and Nails
Anatomy items on this exam link structure to function: which part manufactures the plate, which part carries nerve endings, which part seals out infection. Answers usually hinge on one word in the question.
Nail structures
- Nail plate: keratin; no nerves or blood vessels, so filing or cutting it does not hurt.
- Nail bed: pink tissue beneath the plate, rich in capillaries and nerve endings; attached by the bed epithelium.
- Matrix: generates the plate and extends under the proximal nail fold. It determines thickness and shape, and injury here causes permanent distortion.
- Lunula: the visible whitish portion of the matrix at the base.
- Eponychium: living skin at the proximal fold. Cuticle: the dead, thin layer it sheds onto the plate - the only part you soften and push back.
- Hyponychium: skin under the free edge; the seal against infection. Aggressive cleaning under the free edge breaks it.
- Lateral folds and grooves: the sidewalls that guide growth. Ligaments anchor the plate to bone; if severed, the nail may be lost.
Bones, muscles, nerves
Each hand has 8 carpals, 5 metacarpals, and 14 phalanges. The radius sits on the thumb side, the ulna on the little-finger side. Flexors close the hand, extensors open it. Median, ulnar, and radial nerves supply sensation and movement, and the radial and ulnar arteries form the palmar arches feeding the fingers - which is why a tight band or heavy pressure numbs fingertips.
Growth
Fingernails grow about 3 mm (1/8 inch) a month; toenails roughly a third of that. A fingernail replaces in 4–6 months, a toenail in 9–12. Growth is faster in summer, in children, and on the dominant hand.
Worked example
A client has a deep horizontal groove that has grown to the middle of the plate. Because grooves travel outward with the plate, the injury happened roughly two to three months ago - about halfway through a four-to-six-month replacement cycle. Pushing back the cuticle will not change it.
Traps
- Calling the visible white half-moon the whole matrix.
- Cutting the eponychium because it is labeled "cuticle."
- Believing nails "breathe" - the plate has no blood supply.