The Skin and Appendages

The skin varies greatly in thickness in different breeds. It is remarkably loose on the dorsal aspect of the neck and trunk, where it can be raised in extensive folds. The coat is also subject to extreme variation in length, thickness, color, etc. The hairs are arranged in groups of three to a large extent. The sebaceous glands are best developed in the short and rough haired breeds. They are largest and most numerous at the lips, anus, dorsal surface of the trunk, and sternal region. The sweat glands are relatively better developed in the long and fine haired breeds. The largest are found in the digital pads. Coil glands occur in the skin of the muzzle, the perineum, and the paranal pouches.

On the flexion surface of the carpus and digit there are hairless, cushion-like pads. The carpal pad (Torus carpalis) is situated medial and distal to the accessory carpal bone. The metacarpal and metatarsal pads (Torus metacarpalis, metatarsalis) are the largest and are situated behind the distal ends of the metacarpal or metatarsal bones and the greater part of the first phalanges. They are somewhat heart-shaped, with two unequal lateral lobes, and the apex distal. The metapodiophalangeal joints rest on them when the paw supports the weight. The digital pads (Tori digitales) are oval and are much smaller; they similarly support the second digital joints. That of the first digit is small on the fore limb, and usually absent from the hind limb. Each receives two suspensory bands from the tubercles at the distal end of the corresponding second phalanx. The pads have a basis of fibro-elastic tissue and fat, which is intimately adherent to the skin and is connected by trabeculæ with the bones and tendons. The epidermis is thick and largely keratinized, and is studded with numerous rounded papillæ which are readily seen with the naked eye. The corium has large papillæ, and contains sweat glands and lamellar corpuscles.

The claws (Ungues) correspond in form to the ungual part of the distal phalanges which they inclose. The horn of the claw consists of a body or wall and a sole. The former is strongly curved in both directions, and is compressed laterally. Its root or coronary border fits into the depression under the bony collar or ungual crest of the third phalanx and is covered by skin. The lateral borders converge and inclose the sole anteriorly. The matrix presents a coronary part which bears papillæ only near the coronary border. On the dorsal surface it is thickened to form a ridge, and the corresponding part of the horny covering is thick and maintains the pointed character of the claw. The matrix of the sides of the wall bears small laminæ which converge to the matrix of the sole. The latter is papillated. When the flexor muscles are inactive, the distal phalanges and the claws are maintained in dorsal flexion by two elastic ligaments. These arise from the tubercles at the proximal end of the second phalanx and converge to the dorsal aspect of the ungual crest of the third phalanx.