Skin is the general covering of the entire external surface of the body, including the external auditory meatus and the outer surface of tympanic membrane.
It is continuous with the mucous membrane at the orifices of the body.because of a large number of its functions, the skin is regarded as an important organ of the body.
Function of skin
1- Protection
2- Sensory
3- Regulation of body temperature
4- Absorption
5- Secretion
6- Excretion
7- Regulation of PH
8- Synthesis
9 Storage
10- Reparative
11- Water balance
Surface area
In an adult the surface area of the skin is 1.5-2 sq.metres. in order to assess the area involved in burns, one can follow the rule of nine. Head and neck 9%; each upper limb 9%; the front of the trunk 18%, the back of the trunk 18%;each lower limb18%; and perineum 1%.
The surface area of an individual can be calculated by Du Bois formula. Thus A = W × H × 71.84, where A = surface area in sq.cm, W = weight in Kg, and H = height in cm.
Pigmentation of skin
The colour of the skin is determined by at least five pigments presents at different levels and places of the skin. These are :
1- Melanin (brown)
2- Melanoid
3- Carotene
4- Haemoglobin
5- Oxyhaemoglobin (red)
The amounts of first three pigments vary with the race, age, and part of the body. In white races, the colour of the skin depends chiefly on the vascularity of the dermis and thickness of the keratin. The colour is red where keratin is thin (lip), and it is white where keratin is thick (palms and soles).
Thickness
The thickness of the skin varies from about 0.5 to 3 mm.
Structure of Skin diagram
A). Epidermis
It is the superficial, avascular layer of stratified squamous epithelium. It is ectodermal in origin and gives rise to the appendages of the skin, namely hair, nails, sweat gland and sebaceous glands.
Structurally, the epidermis is made up of a deep germinative zone, comprising :
1- stratum basale
2- stratum spinous
3- stratum granulosum and a superficial cornfield zone
4- stratum lucidum
5- stratum corneum
The cells of the deepest layer proliferate and pass towards the surface to replace the cornfield cells lost due to wear and tear. As the cells migrate superficially, they become more and more flattened, and lose their nuclei to form the flattened dead cells of the stratum corneum.
B) Dermis
Dermis is the deep, vascular layer of the skin, derived from mesoderm. It is made up of connective tissue mixed with blood vessels, lymphatics and nerves . The connective tissue is arranged into a superficial papillary layer and a deep reticular layer. The papillary layer forms conical, blunt projections which fit into reciprocal depression on the undersurface of the epidermis. The reticular layer is composed chiefly of the white fibrous tissue arranged mostly in parallel bundles.
In old age, the elastic fibres atrophy and the skin becomes wrinkled. Overstitching of the skin may lead to rupture of the fibres, followed by scar formation. These scars appear as white streaks on the skin. At the flexure lines of the joints, the skin is firmly adherent to the underlying deep fascia. Dermis is the real skin, because , when dried it makes green hide, and when tanned it makes leather. Its deep surface is continuous with the superficial fascia.
Appendages of skin
1- Nails
Nails are hardened keratin plates on the dorsal surface of the tips of fingers and toes, acting as a rigid support for the digital pads of terminal phalanges. Each nail has the following parts.
A) Root
B) body
C) free border
2- Hair
Hair are keratinous filaments derived from invaginations of the germinative layer of epidermis into the dermis. These are peculiar to mammals and help in conservation of their body heat.
Each hair has an implanted part called the root, a bulb and a projecting part , called the shaft.
Layers of shaft
1- medulla
2- cortex
3- cuticle
Growth of hair
The hair grow at the rate of about 1.5 - 2.2 mm per weak ; their growth controlled by hormones. The life span of the hair various from 4 months to 4 years.
Colour of hair
Colour of hair depends upon the amount and type of melanin pigment.
Sweat glands
Sudoriferous or sweat gland are distributed all over the skin, except for the lips, glans penis and nail bed. These glands are of two types ;
1- Eccrine
2- Apocrine
Eccrine glands
The eccrine glands are much more abundant and distributed in almost every part of the skin. Each gland is a single tube , the deep part of which is coiled into a ball. The coiled part, called the body of the gland , lies in the deeper part of corium or in the subcutaneous tissue. The straight part, called the duct, traverses the dermis and epidermis and opens on the surface of the skin.
Location - the glands are large in the axilla and groin, most numerous in the palms and soles, and least numerous in the neck and back.
Functions - the glands help in regulation of the body temperature by evaporation of sweat, and also hep in excreting the body salts. In dogs, sweat glands are confined to foot pads. Therefore , dogs do not sweat, they pant.
Apocrine Sweat glands
Apocrine glands are confined to axilla, eyelids, nipple and areola of the breast, perianal region, and the external genitalia.
They are larger than eccrine glands and produce a thicker secretion having a characteristics odour. They devlop in close association with hair and their ducts typically open into the distal ends of the hair follicles.
Apocrine Sweat glands Function
Functions ; in animals they produce chemical signals, which are important in courtship and social behaviour.
Sebaceous glands
Locations - sebaceous glands, producing an oily secretion , are widely distributed all over the dermis of the skin , except for the palms and soles. They are specially abundant in the scalp and face, and are also very numerous around the apertures of the ear, nose, mouth and anus.
Structure - sebaceous glands are small and sacculated in appearance, made up of a cluster of about 2-5 piriform alveoli. Sebaceous gland are holocrine in nature, I.e. they produce their secretion by complete fatty degeneration of the central cells of the alveolus, which are then replaced by the proliferating peripheral cels.
Functions - it lubricates skin and protect it forms moisture, desiccation, and the harmful sun rays . Sebum also lubricates hair and prevents them from becoming brittle.
Blood supply
The dermis is vascular while epidermis is avascular. Epidermis cells especially those of stratum basale are supplied nourishment by diffusion.
Nerve supply
There are motor and sensory nerves. The motor nerve fibres are autonomic nerve fibres which are sub motor and vasomotor . Sensory nerve ending in skin are of the following types;
1- free nerve ending in the epidermis for perception of pain.
2- Ruffini's endings are sensitive to heat
3- Krause's bulbs in dermis detects cold. The plexuses around hair follicles detect pain and movement.
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