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).
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.
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.