DIGITS
The specializations of
the fingers frequently have clinical importance. The bones, joints, and tendon
attachments of the fingers have already been described. It remains to add other
specific items of interest or importance (see Plate 4-18).
NAILS
The fingernail is an approximately rectangular horny
plate, the nail plate, composed of closely welded, horny scales, or
cornified epithelial cells. Its semitransparency allows the pink of the highly
vascular nail bed to show through. The nail is partially surrounded by a
fold of skin, the nail wall, and adheres to the subjacent nail bed where
strong fibers pass to the periosteum of the distal phalanx, providing the firm
attachment necessary for the prying and scratching functions of the nail. The
nail is formed from the proximal part of the nail bed, where the epithelium is
particularly thick and extends as far distally as the whitened lunula. Developing
from this nail matrix, the nail moves out over the longitudinal dermal
ridges of the nail bed at a growth rate of approximately 1 mm/wk. Sensory nerve
endings and blood vessels are abundant in the nail bed.
ANTERIOR CLOSED SPACE
To the palmar aspect of the distal phalanx lies the
anterior closed space. Areolar tissue of mixed forms lies in this region. Fiber
bundles surround fatty collections and support the finer arterial and nerve
branching.
More discrete septa of connective tissue fibers pass
from the periosteum of the distal phalanx to blend with the underside of the
dermis. An especially abundant collection of fibers attaches to form the distal
skin crease of the finger and thus serves to bound the anterior closed space of
the finger pad. More proximally at the level of the proximal and distal
interphalangeal joints the palmar skin is held fast by Cleland and Grayson
ligaments during flexion and extension.
SMALL ARTERIES OF DIGITS
The general origin and distribution of the dorsal and palmar
digital arteries have been fully discussed (see Plate 4-18),
and it has been emphasized that the palmar digital arteries are the major
arteries, since they send dorsal terminal branches over the distal and middle phalanges
to supply the dorsum of the fingers and thumb. The dorsal digital arteries are
poorly developed, except in the thumb. The proper palmar arteries are not necessarily of
equal size on the two sides of the digit, although they are essentially so for
the middle and ring fingers. However, in the thumb and the index and fifth
digits, the larger artery is on the median side of the digit; the more
diminutive artery is on the opposite side. These proper palmar digital arteries
have cross anastomoses or transverse interconnections. There is a pair of
proximal transverse digital arteries that anastomoses at the level of the neck
of the proximal phalanx; a pair of distal transverse digital arteries also
anastomoses at the level of the neck of the middle phalanx. These arteries run
close to the bone and deep to the flexor tendons. There is a rich terminal
anastomosis of the palmar digital arteries, which forms a profuse tuft of small
vessels in each finger pad. The proximal edge of this tuft of vessels lies on
the palmar surface of the distal phalanx at about its epiphyseal line.
DIGITAL NERVES
The cutaneous nerves parallel the arteries in course
and distribution. In their course along the fingers, the proper digital nerves
are outside the arteries; that is, as the digit is viewed from the side, the
arteries are within the span of the dorsal and palmar nerves. Cutaneous nerves
are of two types. Included are afferent somatic fibers mediating general
sensation (pain, touch, pressure, and temperature), and efferent autonomic
fibers supplying the smooth muscles, sweat glands, and sebaceous glands.
Both free and encapsulated nerve endings are involved
in various sensations. Of the encapsulated endings, the Meissner tactile
corpuscles are richly represented in the dermal papillae, and pacinian corpuscles
lie in the subcutaneous connective tissue, especially along the sides of the
digits and are quickly adapting receptors responsible for moving touch (tested
by moving two-point discrimination). The slowly adapting receptors (Merkel cell
neurite complexes and
Ruffini end organs) respond to static touch (measured by static two-point
discrimination or with Semmes-Weinstein monofilament testing). The relatively
large size of the proper palmar digital nerves suggests the high density of
nerve endings in the fingers, especially in the finger pads. The tactile
corpuscles are most numerous in the fingertips, less so on the palm, and rare
on the dorsum of the fingers or hand.
