Taber’s Cyclopedic Medical Dictionary Online + Mobile powered by Unbound Medicine. Find 65,000 medical and nursing definitions. Download to iPhone, iPad, and Android. Explore these free sample topics:
-- The first section of this topic is shown below --
1. A rod made of metal, bone, or other solid material used to attach the ends or pieces of broken bones.
2. A horny cell structure of the epidermis forming flat plates upon the dorsal surface of the fingers and toes.
SYN: SEE: onyx; SEE: unguis
A fingernail or toenail consists of a body composed of keratin (the exposed portion) and a root (the proximal portion hidden by the nail fold), both of which rest on the matrix (nail bed). The matrix consists of epithelium and corium continuous with the epidermis and dermis of the skin of the nail fold. The white, crescent area near the root is called the lunula. The epidermis extending from the margin of the nail fold over the root is called eponychium; that underlying the free border of the distal portion is called hyponychium.
A nail grows in length and thickness through cell division in the stratum germinativum of the root. The average rate of growth in fingernails is about 1 mm per week. Growth is slower in toenails and slower in summer than in winter. Nail growth varies with age and is affected by disease and certain hormone deficiencies. The onset of a disease that briefly interferes with nail growth and development may be estimated by measuring the distance of the Beau lines (white transverse lines across the fingernails).
Changes in the nails, such as ridges, may occur after a serious illness or indicate defective nutrition. In achlorhydria and hypochromic anemia, excessively spoon-shaped nails that are depressed in the center may occur. In chronic pulmonary conditions and congenital heart disease, a spongy excess of soft tissue at the base of the nails may be associated with clubbed fingers.
Atrophy may occur as a result of hereditary or congenital tendencies. Permanent atrophy may follow injuries, scars from disease, frostbite, nerve injuries, and hyperthyroidism. Nail shedding is due to the same causes. Fragile or split nails often occur as a congenital condition or may be due to prolonged contact with chemicals or too frequent buffing or filing of the flat surface of the nail during manicuring. In a healthy person brittle nails are usually caused by exposure to solvents, detergents, and soaps. The brittleness disappears when the external causes are avoided. Dry, malformed nails may be due to trophic changes resulting from injury to a nerve or a finger or from neuritis, Raynaud disease, pulmonary osteoarthropathy, syphilis, onychia, scleroderma, acrodermatitis, or granuloma fungoides of the fingers. Beau lines may result from previous interference of nail matrix growth. These lines may be caused by local or systemic conditions. The approximate date of the lesion may be determined because it takes 4 to 6 months for the fingernail to be replaced. Chancre may be suspected if a small indolent ulcer appears near the nail, esp. if indurated and associated with enlarged lymph glands above the inner condyle. Quincke capillary pulsation, indicated by a rhythmic flushing and blanching under the nails, is seen most frequently in aortic regurgitation and often in anemia.
Discoloration of nails is seen in various medical conditions. Black discoloration may be seen in diabetic and other forms of gangrene. Blue-black discoloration is a common condition due to hemorrhage caused by bleeding diseases, such as hemophilia, or trauma. This condition may be painful and can be relieved by drilling a small hole in the nail at the site of the hemorrhage. A dental drill, the heated tip of a paper clip, or a similar rigid wire of small diameter may be used. Brown discoloration may be due to arsenic poisoning. Brownish-black discoloration often indicates chronic mercury poisoning due to the formation of sulfide of mercury in the tissues. Cyanosis of the nails usually indicates anemia, poor circulation, or venous stasis. Green staining of the nail fold or under the nail is associated with the growth of Pseudomonas in a wet area. Slate discoloration is an early manifestation of argyria, and intake of silver should be stopped at once. White spots or striate lesions may be due to trauma and are more frequently seen in women. Transverse white bands in all nails may be a sign of acute or chronic arsenic poisoning or, rarely, of thallium acetate poisoning.
SEE: Mees lines