Deep fascia has many collagen fibres arranged compactly, often in uniform directions so that it may be indistinguishable from aponeuroses. In limbs, where it is especially
well-developed, its fibres are arranged longitudinally and circularly so forming a tough inelastic sheath for the musculature. Muscles are attached to this sheath in both the upper and lower limbs. In the lower limb it is particularly dense as it passes down the outer side of the thigh where it forms the iliotibial tract through which most of the pull of the gluteus maximus muscle is transmitted to the tibia.
In the neck and limbs, laminae of deep fascia run inwards between groups of muscles to connect extensively with bone, often separating different muscle groups with different functions as well as increasing the area of attachment of the muscles.
At wrist and ankle fascia is specialized to form retinaculae which retain tendons running deep to them which would otherwise bowstring out on flexion. In one or two cases (e.g. the digastric muscle) fascia forms a sling which alters the direction of muscle pull even more drastically.
SKIN
Skin is a specialized boundary tissue which forms virtually the entire external surface of the body, being continuous with the mucous membranes lining the respiratory, alimentary, and urinogenital tracts at their respective openings. The skin has many functions. It
minimizes mechanical, thermal, osmotic, chemical, and photic stress, forms a barrier against micro-organisms, regulates heat exchange, and forms a sensory surface equipped with touch, pressure, temperature, and pain receptors. It has good frictional resistance useful in locomotion and handling objects, allows a certain amount of absorption and excretion and serves as a pathway for communication, as in blushing, facial expression, and tactile communication. Vitamin D is synthesized in the skin.
Skin is divided into two distinct parts, the superficial epidermis and the deeper dermis.
The epidermis is a keratinized epithelium of varying thickness, thickest and most heavily keratinized on the sole of the foot and the palm of the hand. Habitual activity, such as holding a pen, digging with a shovel or using scissors, may produce localized thickenings of thick skin called calluses. In other areas, such as the face and the back of the hand, the epidermis is much thinner and less well keratinized. The dermis is made up of connective tissue containing numerous blood vessels, lymphatics, and nerves and is also of variable thickness. Generally the skin is much thicker and tougher on dorsal than on ventral
surfaces. Leather (epidermis + dermis) for shoe soles comes from the back of a cow, glove leather and suede from the belly. In the dermis the bundles of collagen fibres are mostly arranged in parallel rows whose direction is seen as skin creases on the surface. These may be tension lines which are small, irregular furrows dividing the skin into a series of lozenges (easily seen on the back of the hand), or flexure lines which are
associated with regular movements and are conspicuous opposite joints, especially on the palms and soles, where they indicate the points of attachment of the skin to the underlying deep fascia. Papillary ridges (friction ridges) are confined to the hands and feet where they form distinct parallel grooves in arrays determined by underlying dermal papillae. The arrangement of these ridges is under very strong genetic control, as indicated by
hereditary disorders such as Down's syndrome, where a chromosomal anomaly leads to
consistent alterations in their pattern.
An incision made in the direction of a skin crease will cut few collagenous fibre bundles and heal with a minimum of scarring; a cut across the skin creases will leave an unsightly,
bulky mass of scar tissue.
Papillary ridges on the fingers are the basis of dermatoglyphics (fingerprinting) of forensic importance in identifying individuals. Patterns on the palm and sole of the foot may also be used in this way, although these are obviously less convenient.
Appendages
The skin bears nails, hair follicles, sebaceous glands, and sweat glands. Nails are
keratinized plates on the dorsum of fingers and toes. The nail is surrounded on three sides by folds of skin, the nail folds. Hairs grow from follicles, invaginations of the epidermis into the dermis. The follicle is connected to the dermis by an arrector pili muscle, smooth
muscle innervated by the sympathetic nervous system. Contraction of the muscle makes the hair ‘stand on end’ in response to cold or fright. The skin is disturbed by this
manoeuvre to form gooseflesh. Distribution of hair over the body surface is dependent on race, age, and sex. Sebaceous glands associated with hair follicles secrete sebum onto the shafts of hairs, maintaining the flexibility of the hair and helping to waterproof the
surrounding skin. Sweat glands are found all over the body except the vermilion border of the lips and the skin beneath the nails. Sweat is a watery fluid, hypotonic to plasma with about 0.5 per cent solids, the chief one of which is sodium chloride. If exposed to a warm atmosphere, we sweat over the whole body surface. Sweating in response to fear or
anxiety (a cold sweat) occurs chiefly on palms, soles, and axillae, and is produced as a result of stimulation of the sympathetic nervous system which innervates the sweat glands.
At rest the skin leaks water slowly (insensible perspiration); this loss is by osmosis and not through the sweat glands.
Clinical aspects
Nail folds, hair follicles, and sebaceous glands are commonly infected. Infection beneath the nail is called paronychia; infection of a hair follicle is termed a boil. A carbuncle is an infection of the superficial fascia which usually spreads from an infected hair follicle or group of hair follicles.
A burn which destroys the superficial part of the skin will heal from its edges and also by proliferation of the cells of the sweat glands. Healing of a burn which has penetrated deeper can be aided by a skin graft. A split thickness graft removes most of the epidermis from the donor site, leaving the dermis in situ to repair the damage there. A full thickness graft includes the dermis and will slough off if a new circulation is not rapidly established.
The donor site of a full thickness graft is usually covered by a split thickness graft.
Skin colour is determined by at least five pigments, melanin (brown), melanoid (similar to melanin, but differently distributed), carotene (reddish), haemoglobin, and
oxyhaemoglobin (purple and pink respectively). Pigmentation in pale-skinned individuals is variable within certain limits, being influenced principally by exposure to sun and wind.
Melanin helps to protect the deep layers of the skin from the carcinogenic effects of ultraviolet radiation. This is probably the reason why the natives of sunny climates are often dark-skinned. It is possible that the pale skin of Northern Europeans is an adaptation to low levels of sunlight in allowing such small amounts of ultraviolet light as fall upon the skin to penetrate to the sites where vitamin D is manufactured.