The skin is also known as the cutaneous membrane. It is waterproof and expandable within limits. Also
     referred to as simply the integument, the skin is a structural marvel. Most of the skins functions, though
     not all, are protective. It protects deeper tissues from many things:
          Mechanical damage, such as bumps and cuts. The skin is physical barrier containing pressure
          receptors (nerves), which alert the brain to danger
          Chemical damage, such as acid and basic burns. The skin has relatively impermeable keritinized cells
          and pain receptors
          Bacterial damage and infection. The skins secretions are acidic, inhibiting bacteria and forming what
          is known as the "acid mantle". Blood vessels in the skin also contain and move leukocytes of all sorts
          which destroy pathogens and foreign substances
          Ultraviolet radiation, radiation that causes damage. Melanin produced from the melanocytes offer
          protection
          Thermal damage, too much heat or chill. Contains heat, cold, pain receptors to register the threat
          Desiccation, drying out. Keritinized cells are waterproof, holding the water in the body

     The skin aids in the cooling of the body by producing sweat and rushing blood to the surface of the skin
     (flush). The skin also helps to keep the heat in by not letting the skin flush. Sweat is also used to excrete
     urea and uric acid

     The skin modifies cholesterol molecules to make them into vitamin D, using sunlight

     The integument's is made up of two kinds of tissue. The outer dermis, or epidermis is made up of
     stratified squamous epithelium and is full of keratin and hardened, or cornified. The underlying dermis is
     made up of dense fibrous connective tissue. A blister is caused when the two are separated by burning or
     friction. Deep to the dermis is the subcutaneous tissue, essentially areolar tissue containing many fat cells.
     Not really considered part of the skin, it "glues" the skin to underlying organs and tissues. The subcutaneous
     tissue also serves at a sort of shock absorber and insulates to maintain heat

     The epidermis is an epithelial tissue, and accordingly avascular. The epidermis in and of itself contains
     many parts:
          The deepest layer of the epidermis is the stratum germinativum. It lays closest to the dermis and
          contains only dermal cells that receive adequate nourishment diffused from the dermis. These cells
          constantly undergo mitosis, germinating and creating new cells to be pushed upward to replace the
          old ones constantly shed
          As the cells move upward and outward, they become part of the stratum granulosum
          From the stratum granulosum the new skin cells then move to the clear stratum lucidum. From here
          on, they are dead
          The stratum corneum is the upper layer of keritinized, squamous cells. it is waterproof, with no
          circulation or nerve receptors. The
          corneum is shed about every thirty-five to forty-five days
          Melanin is a pigment which ranges in color from pale yellow to black. It is produced by cells called
          melanocytes, found mainly in the stratum germinativum. When exposed to sunlight, which stimulates
          the melanocytes to make more melanin, tanning occurs. The germinativum cells phagocytize, or eat,
          the melanin. The consumed melanin forms a protective pigment shield of sorts over the superficial
          side of the cells nuclei to protect the dna from ultraviolet radiation. Freckles and moles occur when
          melanin accumulates in one spot

     The dermis underlies the epidermis, "epi" meaning above. Leather is the treated dermis of animals such as
     cows. The dense fibrous connective tissue that makes up the dermis has two primary regions:
          The papillary region is the upper of the two dermal regions. It is uneven and have finger-like
          projections on the superior surface, called dermal papillae, which indent the overlying epidermal
          layers. Many dermal papillae contain capillary loops, which provide the nourishment to the lower
          layers of the epidermis. Others contain free nerve endings which are pain receptors, or Meissner's
          corpuscles, touch receptors. On the palms, fingertips, toes and soles of the feet, the papillae are
          arranged in definite swirling patterns to increase the gripping ability. They are genetically determined
          and are well equipped with sweat glands, leaving behind one's distinctive and identifying fingerprints
          The reticular layer is the deepest integumentary layer. It houses blood vessels, sweat glands, oil
          glands, and deep pressure receptors known as Pacinian corpuscles. Also here are many phagocytes
          and throughout the dermis to prevent bacteria from penetrating

     Both collagen and elastic fibers are located throughout the dermis. Collagen is responsible for the toughness
     of the dermis, attract and bind water, helping to keep the skin hydrated. Elastic fibers give it the ability to
     stretch while we are young. As we age, the numbers of both fibers decrease and the subcutaneous fat is
     lost, thus giving way to sagging and wrinkling

     Blood vessels are abundant in the dermis. They help to regulate body temperature. If blood flow is
     restricted for a length of time, decubitus ulcers form. These occur mostly in bedridden patients who are
     not turned or are dragged across the bed a number of times. The skin first becomes blanched at pressure
     points, especially over bony projections. They flush if the pressure is relieved. But if the pressure is not,
     then the cells die, small cracks and breaks appear in the skin, and there is most often permanent damage
     to the blood vessels

     The dermis also contains many nerves and nerve endings with special receptor nerve end organs

     Three pigments contribute to skin color. The amount and kind (yellow, brown or black) melanin, the
     amount of carotene (an orange-yellow pigment from vegetables), and the of oxygen bonded to hemoglobin
     in the dermal blood vessels. When hemoglobin is poorly oxygenated, both the blood and skin take on a blue
     is cast. The condition is called cyanosis. Sometimes cyanosis can be caused by heart failure. In dark skin
     people, the darker melanin shields the cyanosis, but can be seen in the mucous membranes and nail beds:
          Redness or erythema indicates embarrassment, fever, hypertension, inflammation or allergy
          Pallor, paleness or blanching can mean fear, anger, anemia, low blood pressure or impaired blood
          flow
          Yellowing, or jaundice, usually signifies liver problems. The bile coloring leaks out and deposits in
          other tissues
          Black and blue, bruising is bleeding under the skin. The blood had escaped from circulation and has
          clotted in tissue spaces. The clotted masses are known as hematomas. An usual tendency to bruise
          may show a deficiency of vitamin C or hemophilia (bleeder's disease)

     Cutaneous glands are all exocrine glands that release their secretions on the surface of the skin. The fall
     into two groups, being sebaceous glands and sweat glands. They are formed by the cells of the stratum
     germinativum, and push into the deeper tissue. Ultimately, they reside almost totally in the dermis

     Sebaceous glands are oil glands. They are found throughout the skin, except in the palms and the sole of
    the foot. The ducts usually empty into the hair follicles, though some open directly onto the skin surface
     itself. The secretion of these glands is called sebum, a mixture of oily substances and fragmented cells.
     Sebum is a lubricant to keep the skin soft and moist, and prevents hair from becoming brittle and splitting.
     Sebum also contains chemicals to kill bacteria. They become more active during adolescence, when more
     male sex hormones are produced

     Sweat glands, or sudoriferous glands, are spread throughout the skin-- more than 2.5 million per person.
     There are two types of sweat glands:
          Eccrine glands are the more numerous of the two. They produce sweat. Sweat a sterile solution made
          up mostly of water, with some sodium chloride, urea, uric acid, and traces of vitamin C. It is acidic
          with a pH of about four to six, which inhibits the growth of bacteria. The sweat comes out of
          funnel-shaped pores, though the pores commonly referred to on the face are actually the external
          outlets of hair follicles. The eccrine sudoriferous glands are important to the bodies ability to cool
          itself. They are supplied with nerve endings that cause them to secrete whenever external or internal
          temperature rises. When it evaporates off they skin, it carries large quantities of heat with it. One
          can lose up to seven liters of water in this manner
          Apocrine glands are mostly confined to the axillary (armpit) and genital areas of the body. They are
          larger then the eccrine sweat glands, and empty their secretions into hair follicles. Their secretions
          contain fatty acids and proteins in addition to the usual sweat contents. It may have a milky or
          yellowish appearance. The sweat is odorless, but bacteria use the proteins and acids to grow and
          create the characteristic odor we all know and hate. The apocrine glands begin to function during
          puberty, with the influence of androgens (male sex hormones). Their exact purpose is not yet
          known, as they play a minimal role in thermoregulation, but they are stimulated by nerves during
          pain and stress

     Hair is a protective structure. It is produced by a hair follicle, and is made up of dead cells by the time it
     has reached the surface of the skin. The part of the hair enclosed by the follicle is the root, the visible
     part, the shaft. The cells are supplied by the prolific stratum germinativum in the matrix of the hair bulb
     at the inferior end of the follicle. As the cells are pushed out, they become keritinized and die. Each hair
     consists of a core, the medulla, surrounded by a bulky cortex, surrounded in turn by a cuticle only one cell
     layer thick. The cells in the cuticle overlap like shingles to help keep the hair from matting. This it the most
     keritinized layer, providing strength and compacting the deeper layers. At the tip of the shaft, the cuticle
     wears off, causing split ends. Hair color comes form melanocytes in the matrix, and contain all three kinds
     of melanin in differing amounts. When the shaft is oval, the hair is wavy. Flat hair is wavy or kinky. Round
     hairs make up straight tresses. Hair is found everywhere but the palms, soles, and lips of humans. We are
     born with all of the hair follicles we will ever have, but hormones cause hair to grow at different time in
     our lives. Hair follicles are compound structures. The inner sheath is formed of epithelial tissue and forms
     the hair. The outer sheath is dermal tissue. The dermal tissue supplies blood and nourishment to the hair
     root of epithelial tissue and reinforces t. It's nipple-like papilla in the bottom of the follicle provides blood to
     the matrix. The hair is slanted, attached to the arrector pili, which pulls it upright when we are cold or
     stressed, though it is not very useful in humans

     Nails are scale-like modifications of the epidermis. Each terminates in a free edge, with a body and a root.
     The borders of the nails are overlaid by folds of skin called nail folds. The thick, proximal nail fold is most
     commonly known as the cuticle. Stratum germinativum makes up the nail bed, thickening proximal to the
     nail into the nail matrix. Nails, like hair and stratum corneum, are dead, pushed up from the
     germinativum. The nail appears pink only due to the richly vasulated tissue underneath. The exception to
     this is the lunula, a region over the matrix

     Athletes foot is an itchy, red peeling of the skin between the toes from a fungal infection

     Boils and carbuncles are inflamed hair follicles and sebaceous glands on the dorsal neck. Carbuncles are
     composite boils often caused by the bacteria Staphylococcus aureus

     Cold sores, or fever blisters, are small, fluid-filled blisters that sting and itch. They are caused by a herpes
     simplex infection. The virus localizes in a cutaneous nerve, where it remains dormant until activated. The
     trigger may be sun, stress, or a fever. Most often the blisters occur in the oral mucosae

     Contact dermatitis is itchy, red swelling of the skin, progressing into blisters. It is an allergic response to
     chemicals, such as those in poison oak or ivy

     Impetigo is characterized by pink, water-filled lesion that develop a yellowed crust and rupture. It is caused
     by a highly contagious staphylococcus infection, common in school-aged children

     Psoriasis is a chronic condition. Red epidermal lesions are formed, covered by dry, silvery scales. A severe
     case may lead to disfigurement, the cause unknown. Psoriasis may be hereditary, brought on by trauma,
     infection, hormonal changes or stress

     Burns are cellular damage and death caused by heat, electricity, ultraviolet radiation, or chemicals. Burns
     present two major hazards. Fluid is lost, with proteins and electrolytes. This can lead to circulatory shock.
     Lost fluids must be replaced immediately. The volume of fluid lost can be estimated using the nines, how
     much body surface has been burned. This method divides the body into eleven areas, each representing
     nine percent of the body's total surface, with area about the genitals, the perineum, making the last one
     percent. Later, infection is a major concern. The acid mantle is lost, and the deep tissue is exposed.
     Burned skin in sterile for about twenty-four hours. The patients immune system becomes depressed,
     adding to the problem within one or two days after burning

     Burns are classified as first-, second-, and third-degree burns. In fist-degree burns, the skin becomes red
     and swollen. Only the epidermis is damaged. After the initial discomfit, the burn heals quickly, and is not a
     major threat. Sunburns are usually first-degree burns. Second-degree burns damage the epidermis and the
     papillary region of the dermis, and involved blistering. A sufficient amount of epithelial cells are left to
     regrow the damaged tissue. Both are called partial-thickness burns. Third-degree burns are full-thickness.
     The affected area is blanched or blackened. The is little or no pain, because the nerves have been too
     damaged. In such cases, regeneration is not possible, and skin must be regrafted. Burns are critical if over
     twenty-five percent of the body has second-degree burns, over ten percent of the body has third-degree
     burns, or third-degree burns exist on the face, hands, or feet. Facial burns are dangerous to air passages.
     Joint injuries are a difficulty when scar tissue forms, which may limit movement

     Most skin tumors are benign; a wart or neoplasm, for example. Some, however, are not so lucky. Skin
     cancer is the one most common type of cancer, with a tendency to move to other areas of the body. The
     cause of most skin cancers is not known, but one of the more important risk factors is over-exposure to
     ultraviolet radiation. Also to consider are genetics, and predisposition from infection, chemicals, or physical
     trauma

     Basal cells carcinoma is the least malignant and most common of skin cancer. Cells of the stratum
     germinativum are altered so they no longer produce keratin, and enter the dermis and subcutaneous
     tissues. Most often, it appears in sun-exposed areas of the skin as a shining, dome-shaped node. Later, it
     becomes an ulcer with an edge of the same character. Basal cells carcinoma is slow-growing, and seldom
     metastasizes before diagnosis. Ninety-nine percent of the time, the cancer is fully cured by surgery

     Squamous cell carcinoma also arises in the germinativum, but not those cells in direct contact with the
     basement membrane. The lesion shows as a scaly, red papule that slowly changes into an ulcer with a firm,
     raised border. Most often, this cancer appears on the scalp, ears, dorsum of the hands, and lower lip. It
     metastasizes quickly, moving to the near lymph nodes. If caught early and treated with radiation or
     surgery, the chances of curing the growth are good. The cause is believed to be sun-related

     Malignant melanoma is a cancer in the melanocytes. Making up only about five percent of skins cancers, it
     is the most deadly. It can begin wherever there is pigment, but most often occurs in pigmented moles. It
     usually appears as a spreading brown or black patch the metastasizes quickly into surrounding blood vessels
     and lymph nodes. The chances of survival are increased with early diagnosis. The ABCD rule is suggested by
     the American Cancer Society. Look for Asymmetry in pigmented spots. Border irregularity. Colors, such as
     black, brown, tan, even blue or red. Diameter, be cautious of growth

     During the fifth and sixth months of fetal development, an infant is covered with lanugo. Lanugo is a coat
     of downy hair usually shed by birth. When the child is born, the skin is covered in vernix caseosa, a white,
     cheese-like substance produced by the sebaceous glands. It's purpose is to protect the baby's skin in the
     womb. The newborn skin is very thin and translucent. It is quite common for there to be accumulations in
     the sebaceous gland, appearing as white spots on the infants nose and forehead, called milis. Usually, they
     disappear by the third week after birth

     During adolescence, the skin and hair become more oily as sebaceous glands become more active. Acne
     may occur, but usually subsides in early adulthood. Later in life, abrasion, chemicals, sun, wind, and other
     such factors can cause pimples, scales, or various kinds of dermatitis, or skin inflammation. In old age,
     subcutaneous tissue is lost, as are elastic fibers and collagen. The skin become sagging and thin. This is
     sped by the suns harsh ultraviolet radiation. By fifty, the number of hair follicles has dropped by a third,
     and continues to fall, causing baldness, or alopecia. Many men become obviously bald, male pattern
     baldness. The fine, tiny, colorless hairs in the "bald" spots are vellus. Grey hair is caused by a delayed-action
     gene that, when active, causes melanin to be decreased in the hair