NAILS, CLAWS, AND HOOFS

Although these structures enclosing the distal phalanx appear strikingly different at first glance, they are in fact

Figure 10-11 Tactile hairs on the head of the cat.

basically similar. Their origins as local modifications of skin are reflected in their retention of epidermal, dermal, and subcutis layers (though perhaps in greatly altered form). Nails, claws, and hoofs serve primarily to protect the underlying tissues, but each is also used for other purposes, such as scratching or digging or as a weapon. The equine hoof, the most complex, reduces concussion on foot impact, and its elastic nature also aids the return of blood to the heart. Figure 10-18 shows the correspondences between these appendages, each of which presents three parts: wall, sole, and asso­ciated pad. It is only in ungulates that the last forms part of the horny structure; it corresponds with the digital bulb of primates and the digital pad of carnivores.

The nail (wall) of primates grows from the epidermis covering a curved fold of dermis at its base. The epider­mis under most of the nail produces a little horn that helps maintain adhesion as the nail gradually slides distally. The dermis under this rather unproductive

Figure 10-12 A, Schematic longitudinal section of a tactile hair follicle. 1, 2, Internal and external walls of blood sinus; 3, sebaceous gland; 4, root of hair; 5, epidermal wall of hair follicle; 6, nerve ending in wall of blood sinus; 7, blood sinus; 8, dermal papilla. B, Tactile hair follicle calf (Crossmon). 1, Epidermis; 2, sebaceous gland; 3, hair; 4,5, inner and outer hair root sheath; 6, 7, trabeculated blood sinus; 8, inner and outer layer dermal sheath; 9, nerve ending; 10, trabecula.

Figure 10-13 Tylotrich hairs below (top) and above (bottom) tactile elevations (2,2').

portion of the epidermis is gathered into a few low, longitudinal folds (laminae) that interdigitate with cor­responding epidermal laminae; increased dermoepider- mal contact strengthens the bond between the nail and the deeper tissues. The epidermis underlying the free border of the nail produces small amounts of soft “sole horn” (Figure 10-18/2).

The wall of the claw of carnivores can be likened to a nail that has been laterally compressed and so has obtained a sharp dorsal border. Its proximal part and the germinal layer from which it is derived are similarly shaped and are lodged with the associated dermis within the unguicular crest of the distinctively shaped distal phalanx (Figure 10-18, D). The epidermis deep to the wall is minimally productive. The dermis that covers the unguicular process fuses with the periosteum, and as with the primate nail, longitudinal interdigitations between dermal and epidermal laminae strongly bond the claw to the dorsal border of the bone. The space between the free margins of the wall on the undersur­face of the unguicular process is filled with flaky “sole horn” (Figure 10-18/5).

The wall of the horse’s hoof is also strongly curved, and the sides are sharply inflected to form the so-called bars (Figure 10-19, E/2"). The space between the bars is occupied by the frog, the part of the footpad that makes contact with the ground. The sole horn that fills the ground surface between wall and frog meets the wall at a junction known as the white line (zona alba; Figure 10-19/5). The wall grows distally from the epidermis

Figure 10-14 Footpads of a bear: forelimb (left), hindlimb (right). 1, Digital pads; 2, metacarpal pad; 3, metatarsal pad; 4, carpal pads; 5, tarsal pad, fused with the metatarsal pad.

Figure 10-15 Footpads of canine forelimbs and hindlimbs (A, A') and of feline forelimb (B).

Figure 10-16 Palmar surface of foot of the pig (left) and of a cow (right). 1, Bulb (digital pad) of hoof; 2, sole of hoof; 3, wall of hoof; 4, hoof of accessory digit; 5, rudimentary hoof of dewclaw.

over a bulging (coronary) dermis studded with numer­ous papillae directed toward the ground. The epidermis covering these papillae produces horn tubules that run distally, toward the weight-bearing margin of the wall. The tubules are embedded in less structured intertubu­lar horn formed by the epidermis over the interpapillary regions of the dermis; the combination of horn types gives the tissue a finely striated appearance. The (laminar) epidermis deep to the wall is again only mini­mally productive. It is arranged as several hundred well- formed laminae that tightly interdigitate with an equal number of dermal laminae (p. 611), bonding the wall to the underlying distal phalanx. One should remember that this is a living bond that allows the wall to slide gradually toward the ground where its distal border is worn away. A band of soft horn (periople) lies over the external surface of the wall near its junction with the skin (Figure 10—20/7). It descends with the wall and dries to a protective glossy layer. The band widens at the back of the hoof, where it covers the bulbs of the heels and part of the frog.

The hoofs of ruminants and the pig, although resem­bling those of the horse in principle, differ in several respects: the wall is sharply bent to form a dorsal border (like that of the claw); the footpad (bulb) is relatively large and furnishes the entire caudal part of the hoof (Figure 10-19, B/4); the sole between the bulb and wall is small; and the interdigitating laminae are less devel­oped (Figure 10-21/2).

In all species, periods of disturbed or lessened horn production create ridges on the wall parallel to the for­mative region at the junction with the skin (Figure 10-20/2).

Fuller accounts of these specializations are found in the appropriate later chapters.