Upon the tips of human fingers one can easily see with the naked eye peculiar fine ridges, called friction ridges because they aid to a certain extent in preventing the fingers from slipping when brought into contact with objects. They are arranged mostly at right angles to the direction in which there is the greatest tendency to slip. Sweat glands that open upon them, like craters along the peaks of tiny volcanic mountain chains, provide moisture, bringing about much the same result as when a workman “spits on his hands” to secure a better grasp.
Since friction ridges appear only on those areas that come habitually into contact with objects, they are particularly developed on the palmar and plantar surfaces of the hands and feet of man and other primates, and also on the concave side of the prehensile tail of the long-tailed American monkeys (Fig. 205). They are absent from the middle of the back, forehead, and rim of the ear, and other regions not employed in taking hold of things.
A histological examination reveals the fact that the ridges and furrows of the epidermis in the friction areas match corresponding downgrowths into the underlying derma. Furrows between papillary ridges should not be confused with the many wrinkles and folds that beset the skin all over the body.
Friction areas are particularly associated with padlike epidermal elevations, or tori, that originally appear on the palms and soles. Typically there are ten of these elevated tori on each hand or foot, namely, five digital areas forming the balls of the fingers and toes; three inter digital areas on the palm or sole near the base of the digits; one thenar; and one hypothenar area at the posterior part of the palm or sole on the side of the big digit and the little digit respectively (Fig. 206). Although present as distinct elevations throughout life on the feet of certain mammals, the mouse for example, and also on the hands and feet of the human embryo, tori as such disappear in adult man, since, as the human embryo grows older, these elevations or pads become less pronounced and are eventually flattened to form the friction areas.
The various minute patterns which the ridges of the friction areas assume are all definitely established before birth and retain their individuality, except for slight increase in size, throughout life. It has been demonstrated that when friction ridge patterns are destroyed by searing or by sandpapering the finger tips, the old patterns are restored upon subsequent growth of new epidermis.
Since the details of the patterns are unlike, not only in different persons but also on the twenty fingers and toes of the same person, they furnish an excellent means for personal identification. Just as primitive peoples in the past have frequently employed indelible tattoo marks in order to distinguish themselves from their fellows, so friction ridge patterns, which have been called “nature’s tattoo marks,” are made to serve a like purpose.
The patterns may be roughly classified in general types, namely, whorls, loops, composites, and arches, as indicated in Figure 207. Loops may be ulnar or radial according to whether they stream outward toward the ulnar (little finger) or the radial (thumb) side, while arches may be simple, as in the figure, or if more pronounced, tented arches. Combinations of these types upon the fingers of both hands taken together, and the infinite variety in the minutiae that each type reveals upon careful scrutiny, make possible an almost unlimited subdivision and classification. Thus, it has come about that finger-print codes have been worked out, which may even be telegraphed or radioed from one part of the world to another in the interests of personal identification.
By reason of the fact that finger prints are easily made and kept on file, they may be utilized conveniently in a great variety of ways. Upon a bank cheque, passport, or non-transferrable documents of any kind, for example, such a personal imprint furnishes a unique signature which cannot be forged. In the case of soldiers, sailors, the personnel of large industrial plants, voters, babies at maternity hospitals, inmates of institutions, criminals, undesirable immigrants once rejected, dead bodies recovered from disastrous catastrophes or accidents, aphasia victims, and in many other instances, finger prints offer a simple and invaluable means of establishing identity (Fig. 208).
Since Galton’s pioneer work in England, and the appearance of Mark Twain’s whimsical classic, in which the imagination of the story teller anticipated the later applications of science, the serious study and utilization of the ineffaceable friction ridges has developed into a real science by itself (Dactyloscopy) with a considerable and growing bibliography. It is now known that two widely separated peoples, the Chinese and the Babylonians, in very early times made use of finger-print signatures.