fingerprints

fingerprints In the minds of many people, fingerprints are significant only for the identification of criminals. But the skin corrugations that produce fingerprints are a functionally important part of the structure of the finger pads, not only in human beings, but also in a number of other mammals, especially primates, which use prehensile hands (and feet) for active exploration of surfaces and for the fine manipulation of objects.

Fingerprints form naturally during the development of the human fetus, starting about 13 weeks after conception. The inner surfaces of the fingers and the palms of the hands, which are covered with hairless (glabrous) skin, develop tiny ‘pods’, which are the precursors of the pores of sweat glands. These pods, or ridge units, expand and coalesce with neighbouring pods, producing roughly linear ridges, with the sweat pores distributed along their crests, raised above the surface of the surrounding skin. These form the familiar parallel and swirling ridges and intervening furrows, the exact pattern of which is determined by complex, irregular stresses in the skin. They can be seen not only on the pads of the fingers and thumb, but over much of the glabrous skin on the undersurface of fingers and toes, and on the palms and soles of the hands and feet. (A little talcum powder dusted over the surface of the skin makes the pattern more easily visible.)

The ducts of the sweat glands open through the pores on the crests of the ridges. The moistening of the ridges, combined with the texture of the corrugations, increases friction when in contact with objects and hence improves grip. The regions of glabrous skin that have these epidermal ridges are especially richly supplied with cutaneous sensory nerves. These include large fibres that terminate in specialized endings that are sensitive to mechanical stimulation, in particular structures called Merkel's discs and Meissner's corpuscles, which are acutely sensitive to touch and to low-frequency vibration of the skin, respectively. The individual nerve fibres innervating the finger pads branch over areas of skin that are tiny compared with similar classes of fibres in other parts of the body surface. Thus the receptive field of each such fibre in the finger pads (the area of skin over which the application of an appropriate stimulus will cause an individual nerve fibre to respond) can be smaller than 1 mm². The skin of the fingertips therefore excels in its capacity to detect and discriminate the texture and three-dimensional shapes of surfaces. As the fingertips are moved over a non-smooth surface (when a blind person reads embossed Braille characters, for example), the resulting pattern of impulses generated in these nerve fibres and transmitted up to the brain provides remarkably acute tactile perception.

Serendipitously, the sweat secreted onto the skin ridges leaves an oily image of the pattern of corrugations on any surface that is touched. Since the exact forms of the corrugations are unique to each individual, and do not alter from birth to death (unless the skin is badly injured), fingerprints provide an infallible method of identification.

History

The discovery of the uniqueness of fingerprints is very ancient: the Chinese and Assyrians used them on legal documents more than 2000 years ago. The great Czech anatomist, Jan Evangelista Purkinje (who gave his name to the main class of nerve cells in the cerebellum), studied the pattern of skin ridges and, in 1823, suggested a method of classification. The first Police Fingerprint Bureau was established in Argentina early in the 1890s by a Croatian immigrant, Juan Vucetich, whose system of classification is still used in South America. But the best-known system was devised a few years later by a police official, Khan Bahudur Azizul Haque, of the Bengal police, under the direction of Edward (later Sir Edward) Henry. When Henry was appointed Assistant Commissioner of Police for the Metropolis in London at the turn of the twentieth century, he established the first British Fingerprint Bureau at New Scotland Yard. The Henry System, based on prints of all the fingers, spread throughout the world. It quickly replaced the Bertillon system for identification, devised in 1879 by the French criminologist Alphonse Bertillon, which involved anthropometric measurement of parts of the body and detailed records of personal features, such as scars and eye colour.

The appearance of fingerprints

A record of an individual's fingerprints is made by inking the pads of the fingers and thumb and pressing or rolling them onto paper or some other suitable material. The most distinctive overall fingerprint patterns (arches, loops, and whorls) occur on the pads of the fingers, thumbs, and toes. The one illustrated is a magnified image of a ‘loop’, the commonest such pattern, which almost everyone has at some point on their skin. The sweat pores appear as little white dots at intervals along the ridges. Within such overall patterns are individual features known as ‘ridge characteristics’, ‘ridge detail’, or ‘minutiae’. In places, the parallel ridges split into two (a fork or bifurcation), stop dead (a ridge ending), or divide into two and then join up again (a lake or enclosure). These features were studied and named by the British anthropologist and founder of the science of eugenics, Sir Francis Galton. It is such features (marked in the figure) and their proximity to one another that define the unique individuality of fingerprints. The prints of each finger pad typically contain about 100 such minutiae.

In many people, the overall macro-patterns on corresponding fingers of the left and right hands are roughly mirror images of each other. But it is possible to have a different feature pattern on every digit. Although the gross patterns (arches, loops, and whorls) may be passed on through family lineage, the individual ridge details are not. Since these minutiae are presumably not directly genetically determined, they differ between identical twins, and presumably would not be the same even in the clone of an individual.

Minor cuts and abrasions, and some skin diseases (e.g. psoriasis and eczema), may temporarily disturb the ridged skin features, but after healing the structure is exactly the same as before. More serious injuries or burns, involving the deeper layer of the skin (the dermis), can damage the cells that are responsible for regeneration of the skin, and leave a scar within which the ridge pattern is lost or changed. However, identity can still be established from surviving features outside the area of damage. A notorious American criminal, Roscoe Pitts, eliminated the prints of his fingertips by having them sewn into incisions in his chest until new skin grew over the ridge pattern. But he was later identified from prints of his palm, left at the scene of a crime!

Use in crime detection

When a surface is touched by a human hand, oily sweat is deposited from the skin ridges. This ‘latent print’ is often invisible to the naked eye, but can be revealed by a light dusting of powder, using a fine brush. Nowadays, aluminium flake powder is usually applied with a fibreglass brush, but a whole battery of physical and chemical processes is also available to develop prints on almost any surface.

Fingerprints collected at the scene of a crime are then matched against those taken from previously convicted criminals (now held as graphics files on computer databases), or from suspects. Although there have been encouraging developments in automated, ‘expert system’ computerized methods for the matching process, identifications are still made exclusively by experienced Fingerprint Officers.

Fingerprints are not the only means of identifying individuals. Voiceprints, lip prints, ear prints, glove prints, and DNA profiling provide additional or alternative methods for recognition.

S. E. Haylock

See also DNA fingerprinting; skin; voiceprint.