The component parts of the nail apparatus are shown in Figure 1.1. The
rectangular nail plate is the largest structure, resting on and firmly attached to
the nail bed and the underlying bones; it is less firmly attached proximally, apart
from the posterolateral corners. Approximately one-quarter of the nail is covered
by the proximal nail fold, while a narrow margin of the sides of the nail plate is
often occluded by the lateral nail folds. Underlying the proximal part of the nail
is the white lunula (‘half-moon’ or lunule); this area represents the most distal
region of the matrix. The natural shape of the free margin of the nail is the same
as the contour of the distal border of the lunula. The nail plate distal to the
lunula is usually pink owing to its translucency, which allows the redness of the
vascular nail bed to be seen through it. The proximal nail fold has two epithelial
surfaces, dorsal and ventral; at the junction of the two the cuticle projects
distally on to the nail surface. The lateral nail folds are in continuity with the
skin on the sides of the digit laterally, and medially they are joined by the nail
bed.
The nail matrix can be subdivided into proximal (or dorsal) and distal (or
intermediate) sections, the latter underlying the nail plate to the distal border of
the lunula. It is now generally considered that the nail bed contributes to the
deep surface of the nail plate (ventral matrix), although this thin, soft, deep
component plays little part in the functional integrity of the nail plate in its distal
part. At the point of separation of the nail plate from the nail bed, the proximal
part of the hyponychium may be modified as the solehorn. In hooved animals
this is the site of hard keratin hoof formation—it may also be the source of hard,
distal subungual hyperkeratosis in diseases such as psoriasis and pachyonychia
congenita. Beyond the solehorn region the hyponychium terminates at the distal
nail groove; the tip of the digit beyond this ridge assumes the structure of the
epidermis elsewhere.
When the attached nail plate is viewed from above, several distinct areas may
be visible, such as the proximal lunula and the larger pink zone. On close
examination two further distal zones can often be identified: the distal
yellowish-white margin, and immediately proximal to this the onychodermal
band. The latter is a barely perceptible, narrow transverse band 0.5–1.5 mm
wide. The exact anatomical basis for the onychodermal (onychocorneal) band is
not known but it appears to have a separate blood supply from that of the main
body of the nail bed; if the tip of the finger is pressed firmly, the band and an
area just proximal to it blanch, and if the pressure is repeated several times the
band reddens.

Figure 1.1

(a), (b) Nail apparatus structures; (c) longitudinal nail biopsy
section, oriented to equate with (b).