With the exception of the amphibians, the tips of the digits in those vertebrates that have fingers and toes are reinforced by hard integumentary structures, either claws, nails, or hoofs. Although amphibians do not have true claws, a thickening of the epidermal corneal layer at the ends of the fingers and toes is a prophecy of claws to come later in the vertebrate series. In the African toad Xenopus, and the Japanase salamander Onychodactylus, these epidermal thickenings assume the definiteness of actual claws on some of the digits.
The typical claw of a reptile (Fig. 179, A) may be regarded as being made up of two scalelike horny plates, dorsal and ventral, so placed as to converge to a point at the end of the digit. The convex dorsal plate, unguis, is rounded in two directions, towards the tip and towards the lateral margins. The smaller ventral plate, subunguis, which is pinched in between the lateral edges of the unguis, is more flattened and of a less dense texture. Both structures are produced entirely by the Malpighian layer of the epidermis.
In most cases the claws of birds are confined to the toes, although Archaeopteryx, the oldest known bird, had three finger-claws on each wing, while some existing types of running birds (Ratitae), have claws on the degenerate first and second fingers. The young hoactzin, of British Guiana, also has claws on its wings which enable it to venture from the nest and scramble about in trees on “all fours” like a lizard (Fig. 180).
In general pattern the claws of a bird are reptilian, although assuming a wide variety of forms adapted to correspondingly different functions. The sharp, slender claws of a woodpecker or chimney swift are designed for clinging to rough surfaces; the blunt stout claws of the domestic fowl for scratching; the hook-like talons of hawks and owls for grasping prey; while the long straight claws of the Mexican jacana (Fig. 181), on the ends of elongated toes, enable this tropical species, in pursuit of its insect prey, to ski over unstable lily pads that float on the surface of the water.
Certain species of the grouse family (Tetraonidae) undergo a periodic ecdysis and renewal of the entire claw, a reminder of the changes undergone by other epidermal structures, such as the thimble-like caps on the horns of pronghorn antelopes, or the corneal layer of a snake’s skin.
Mammalian claws cover the terminal bony phalanx of each digit. They consist (Fig. 179, B) of the unguis and subunguis of the reptilian claw, and in addition a terminal pad, or cushion, just behind the claw on the ventral side of the digital tip. Since the animal bears its weight on these cushions, the corneal layer that clothes them is considerably thickened in consequence. Usually the dominant unguis becomes laterally compressed and curved down to a point, with the result that the subunguis is much reduced.
The claws of a cat are sharp and retractile within a protective sheath (Fig. 182), thus being kept unworn to be extended for use only in emergencies. The claws of a dog are less pointed and more exposed at all times, particularly so when it runs, for then they may come into contact with the ground, despite the presence of pads.
Bats and sloths have claws developed into elongated hooks which, although making locomotion on the ground awkward and difficult, are very useful when these animals hang themselves upside down from branches in trees, as is their habit.
There are two striking modifications of mammalian claws, namely, hoofs and nails. In the first case the unguis thickens enormously into the shoelike hoof, which is so convex that its edges reach all the way around to the ventral side and come into contact with the ground. In the other case the unguis becomes flattened into the conspicuous nail, and the subunguis shrinks to a narrow insignificant rudiment under the projecting eaves of the nail, while the terminal pad becomes transformed into a sensitive ball, occupying the entire ventral aspect of the digital tip.
In a typical hoof (Fig. 179, E) such as that of a horse, the subunguis fills in ventrally the space between the lateral edges of the unguis, and the pad forms into a tough mass of material behind it, called the “frog,” that serves somewhat the purpose of a rubber heel on a shoe. Hoofs, like the thick-soled shoes of a traffic policeman, are useful in supporting the heavy weight of the body in such animals as stand for long periods of time and so need a firm foundation to bear up their weight. The heavy elephant, which has a hoof on each toe, is particularly well provided with “rubber heels” (Fig. 183).
The surefootedness of hoofed animals, like the donkey and mountain goat, is in part due to the fact that the softer subunguis wears away faster than the harder outer edge of the unguis, thus insuring a constantly well-shod foot with a sharp hard edge, in spite of destructive contact with rocky ground.
Nails occur in man and other primates where they reinforce and protect the sensitive finger pads which play such an incalculable role in life. A person who attempts to pick up a pin, for example, with the fingers encased in gloves quickly realizes how much these structures can aid.
The human nail, which corresponds to the unguinal part of the claw flattened out, is made up from closely compacted epidermal cells of the stratum lucidum, that is, the lifeless remains of what were once Malpighian cells. During its growth the distal part of the nail is continually advanced toward the tip of the finger or toe by additions from a thickened germinal matrix of Malpighian cells at its base, the position of which, particularly on the thumb, is marked by a white half moon, or lunula (Fig. 184).
The pinkish color of the nail, aside from the lunula, is due to its translucency which allows the blood beneath to show through. The lunula is white because the mass of Malpighian cells forming the nail-bed is so thick that the blood does not show through. Upon pressure the blood in the underlying capillaries can be made to retreat, leaving the entire nail white like the lunula so long as the pressure is maintained. Transient white flecks sometimes appear in fingernails, due to accidental air-spaces imprisoned between the dead scalelike remains of the stratum lucidum.
The whole nail pushes out through a superficial corneal layer of the epidermis, leaving a ragged margin of corneum, the eponychium (Fig. 185), that may be seen encroaching upon the lunula and along the sides of the unguis. Under the free outer edge of the nail, where the continuity of the corneum is again broken, there is a narrow transitional region which is all that remains of the subunguis of the reptilian claw. Dirt collects here. The refinement of manicuring consists largely in attending to the ragged frame of corneum through which the nail projects.
The rate of growth of human nails is roughly an inch in six months, the rate varying with the general condition of health. A recent illness may be recorded by transverse depressions on the nails.
If human nails were never trimmed or broken, they ought theoretically to attain a length of over ten feet by the time one reaches the allotted age of three score years and ten.
In the human fetus the twenty nails first appear as terminal amphibian-like epidermal thickenings, at about the ninth week. By the twelfth week they are perfectly formed, but it is considerably later when they finally migrate into their dorsal position (Fig. 186).
The transition from the laterally compressed claws of most mammals to the flattened nails of primates is strikingly illustrated in certain lemurs which have claws on some of their digits and rounded nails on others (Fig. 187).