The skin of vertebrates consists of an outer layer, the epidermis, which develop from the ectoderm; and an underlying mesodermal layer known as the dermis. Below the dermis are deposits of adipose tissue ( fat ).

Skin of man.

• The skin in man varies in thickness from less than 0.5 mm to 3 or 4 mm in different parts of the body.
• Under proper conditions the skin is germproof and serves as an effective barrier against disease organisms. It is practically impervious to water and under exposure to ultraviolet rays produces vitamin D.
• The epidermis is arranged in layers of cells. The most active cells of the epidermis is the single layer of columnar cells located perpendicular to the dermis. Here these cells are actively dividing and pushing the previous cells produced to the surface. This area is called the stratum germinativium. These cells eventually reach the surface. They are dead and scaly and leave the body as flakes. This area is called the stratum corneum.
• Keratin, a waterproof protein is found in the stratum corneum. Keratin is extra thick on the soles of the feet and the palms of the hands.
• The dermis is sometimes called the true skin, and is best developed in mammals. It is not composed entirely of cells but of connective tissue fibers extending in all directions.
• Dermis is made up of 2 layers. The outer papillary layer and the reticular layer.
• The epidermis and dermis are arranged in a mountain valley arrangement. The papillary layer form large tooth-like projections. This causes the epidermis to filter down and fill the spaces. This tongue and groove arrangement allows the skin to be smooth as well allowing it to provide the maximum amount of friction with a surface.
• Pigment in vertebrates occurs in cells called melenocytes. Melanin is a brown pigment found in these cells. Melenocytes are found in the lower epidermis of the skin. The cell loses the pigment as it move toward the surface. In true albinos there is no skin pigment.
• Skin color depends on three factors: skin is yellow in color, blood vessels give it a reddish hue; the pigment melanin in different concentrations cause various shades of brown. The actual amount of pigment is relatively small. It has been estimated that there is about 1 gram of pigment in the entire skin of an average Negroid.
• The dermis contains sweat glands, sensory receptors, lymph glands, blood vessels, nerves, and hair follicles.
• Hair grows from the follicle imbedded in the dermis. A sebaceous gland ( oil gland ) and an arrector muscle are attached to each follicle. The area above the follicle (hair ) is an accumulation of dead cells.
• Four factors determine hair color: color of pigment found in the cortex, amount of pigment, character of hair surface, amount of air contained within the inner space of the hair.
• Finger nails grow from the root of the nail just behind the nail under the skin.