Overview
The cat's claw (unguicula) is a modified skin structure: a sheath of hard keratin wrapping the ungual process of the distal phalanx. At its base the ungual crest of the phalanx forms a bony fold that covers the proximal edge of the claw. Cat claws are retractile: at rest the dorsal elastic ligaments hold the distal phalanx folded beside the middle phalanx, and the claw stays hidden in the fur, without wearing down as the cat walks. When the cat contracts the deep digital flexor, the distal phalanx rotates and the claw comes out.
The thoracic limb has five claws, including that of the dewclaw (digit I), which does not touch the ground. The pelvic limb normally has four.
Structure
- Wall: forms the dorsal and side surfaces; it is curved and strongly compressed laterally, which makes it sharp.
- Sole: palmar or plantar surface of softer keratin.
- Claw dermis: highly vascular, innervated connective tissue between the phalanx and the keratin; it is the living part. In unpigmented claws it shows as a pink zone.
- Growth in layers: the claw grows continuously from its base. The old outer layers come off as sheaths when the cat scratches, leaving a new, sharp claw underneath.
Function
Holding prey, climbing, defence and marking: by scratching surfaces the cat leaves visible marks and scent from the glands of the pads.
Clinical relevance
- Ingrown claws: in older or inactive cats the claws are sharpened less, overgrow and can pierce the pad; this is common in the dewclaw and in polydactyl cats.
- Nail trimming: pressing the pad extends the claw and lets only the tip be cut, distal to the pink dermis.
- Onychectomy: removing the claws requires amputation of the distal phalanx; it is banned in many countries. The alternatives are regular trimming, scratching posts and nail caps.
- Paronychia: infection or inflammation of the claw fold. Paronychia with caseous exudate in several digits suggests pemphigus foliaceus.
- Lung-digit syndrome: digital metastases of a primary lung carcinoma present as swelling and deformity of the claw and digit.