FOOTPADS

The footpads (tori) are the cushions on which animals walk. They are covered by a naked, densely cornified epidermis (see Figure 10-2). The dermis is unremark­able, and the bulk of their substance is provided by a thick, resilient subcutis, an admixture of collagenous and elastic fibers interspersed with adipose tissue.

Footpads are best developed in plantigrade mammals (e.g., bears), in which digital, metacarpal (metatarsal), and carpal (tarsal) pads are all present (Figure 10-14). In the digitigrade dog and cat, only digital and meta­carpal (metatarsal) pads make ground contact; there is a carpal pad of no obvious use but no corresponding tarsal pad (Figure 10-15).

Only digital pads are functional and in contact with the ground in ungulates, in which they are (generally) incorporated in the hoof, providing the features known as the bulb in ruminants and pigs and the more complex frog in horses. The bulbs of the pig are soft and well set

Figure 10-10 A-C, Hair follicles of the dog. A, Simple follicle present shortly after birth. B, Follicle present during the first few months after birth. C, Complex adult follicle; the primary hair is surrounded by several secondary hairs. D, Scanning electron micrograph of adult canine skin; note one or two follicles without primary (guard) hairs. E, "Naked" skin of a pig with sparse primary hairs (bristles) and surface debris. 1, Primary hair follicle; 2, sebaceous gland; 3, duct of sweat gland; 4, secondary hair follicle; 5, arrector pili muscle.

off from the sole (see further on); in ruminants they are harder, though less so than other parts of the hoof (Figure 10—16/7).

The digital cushion (pulvinus digitalis) deep to the frog of the horse consists of an apex and a base. The apex lies deep to the horny frog on the ground surface of the hoof (Figure 10-17∕√), while the base helps shape the palmar (plantar) surface, forming the swellings at the heels. These, the bulbs of the heels (Figure 10-17/5), do not make contact with the ground and are covered by periople, the softer horn produced at the junction of the skin with the wall of the hoof. The horse, unlike the other domestic ungulates, also has rudimentary meta­carpal (metatarsal) pads (“ergots”; Figure 10-17/2) embedded in a tuft of hair behind the fetlock joint and vestigial carpal (tarsal) pads (chestnuts; Figure 10-17/1,1').

The subcutis of the canine footpads, porcine bulbs, and equine frog contains sweat glands whose ducts channel through the thick, cornified epi­dermis. The secretions function as territorial or trail markers.