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Skin, outer body covering of an animal. The term skin is commonly used to describe the body covering of any animal but technically refers only to the body covering of vertebrates (animals that have a backbone). The skin has the same basic structure in all vertebrates, including fish, reptiles, birds, and humans and other mammals. This article focuses primarily on human skin.

The skin is essential to a person’s survival. It forms a barrier that helps prevent harmful microorganisms and chemicals from entering the body, and it also prevents the loss of life-sustaining body fluids. It protects the vital structures inside the body from injury and from the potentially damaging ultraviolet rays of the sun. The skin also helps regulate body temperature, excretes some waste products, and is an important sensory organ. It contains various types of specialized nerve cells responsible for the sense of touch.

The skin is the body’s largest organ—that of an average adult male weighs 4.5 to 5 kg (10 to 11 lb) and measures about 2 sq m (22 sq ft) in area. It covers the surface of the body at a thickness of just 1.4 to 4.0 mm (0.06 to 0.16 in). The skin is thickest on areas of the body that regularly rub against objects, such as the palms of the hands and the soles of the feet. Both delicate and resilient, the skin constantly renews itself and has a remarkable ability to repair itself after injury.




Structure of the Skin

The skin consists of an outer, protective layer (epidermis) and an inner, living layer (dermis). The top layer of the epidermis is composed of dead cells containing keratin, the horny protein that also makes up hair and nails.

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The skin is made up of two layers, the epidermis and the dermis. The epidermis, the upper or outer layer of the skin, is a tough, waterproof, protective layer. The dermis, or inner layer, is thicker than the epidermis and gives the skin its strength and elasticity. The two layers of the skin are anchored to one another by a thin but complex layer of tissue, known as the basement membrane. This tissue is composed of a series of elaborately interconnecting molecules that act as ropes and grappling hooks to hold the skin together. Below the dermis is the subcutaneous layer, a layer of tissue composed of protein fibers and adipose tissue (fat). Although not part of the skin itself, the subcutaneous layer contains glands and other skin structures, as well as sensory receptors involved in the sense of touch.




About 90 percent of the cells in the epidermis are keratinocytes, named because they produce a tough, fibrous protein called keratin. This protein is the main structural protein of the epidermis, and it provides many of the skin’s protective properties. Keratinocytes in the epidermis are arranged in layers, with the youngest cells in the lower layers and the oldest cells in the upper layers. The old keratinocytes at the surface of the skin constantly slough off. Meanwhile, cells in the lower layers of the epidermis divide continually, producing new keratinocytes to replace those that have sloughed off. As keratinocytes push up through the layers of the epidermis, they age and, in the process, produce keratin. By the time the cells reach the uppermost layer of the epidermis, they are dead and completely filled with the tough protein. Healthy epidermis replaces itself in a neatly orchestrated way every month.

Scattered among the keratinocytes in the epidermis are melanocytes, cells that produce a dark pigment called melanin. This pigment gives color to the skin and protects it from the sun’s ultraviolet rays. After being produced in the melanocytes, packets of melanin called melanosomes transfer to the keratinocytes. There they are arranged to protect the deoxyribonucleic acid (DNA), or genetic material, of the keratinocytes.

All people have roughly the same number of melanocytes. Differences in skin color, such as that between light-skinned people of European descent and dark-skinned people of African descent, result from differences in the amount of melanin produced and how melanosomes are arranged in the keratinocytes. Particularly in people with light skin, melanin sometimes accumulates in patches, forming freckles, age spots, or liver spots.

In people of almost any skin tone, exposure to the sun causes melanocytes to increase their production of melanin to help protect the skin. This process results in a darkening of the skin tone to form a suntan. The suntan fades when keratinocytes containing the extra melanin are sloughed off. A type of melanin called pheomelanin makes redheaded people more sensitive to the sun. A total lack of melanin, a genetic condition called albinism, makes people extremely sensitive to the sun. People with albinism can be members of any race and have very light skin, hair, and eyes.

The epidermis also contains a type of immune cell known as a Langerhans cell. Produced in the bone marrow, Langerhans cells take up sentrylike positions in the epidermis, where they help cells of the immune system recognize potentially dangerous microorganisms and chemicals.

Another cell in the epidermis is the Merkel cell, found in sensitive, hairless areas such as the fingertips and lips. Located in the deepest layer of the epidermis, Merkel cells contact nerve endings in the dermis below and function as a type of touch receptor.




Unlike the epidermis, the dermis or lower layer of the skin is richly supplied with blood vessels and sensory nerve endings. The dermis also contains relatively few cells compared to the epidermis—instead, it is made up mainly of fibrous proteins and other large molecules.

The main structural component of the dermis is a protein called collagen. Bundles of collagen molecules pack together throughout the dermis, accounting for three-fourths of the dry weight of skin. Collagen is also responsible for the skin’s strength. Another protein in the dermis, elastin, is the main component of elastic fibers. These protein bundles give skin its elasticity—the ability to return to its original shape after stretching. Collagen and elastin are produced by cells called fibroblasts, which are found scattered throughout the dermis.

The upper part of the dermis is known as the papillary layer. It is characterized by dermal papillae, tiny, fingerlike projections of tissue that indent into the epidermis above. In the thick skin on the palms and soles, the epidermis conforms to the shape of the underlying dermal papillae, forming ridges and valleys that we know as fingerprints. These ridges provide traction that helps people grasp objects and surfaces.

Some dermal papillae contain touch receptors called Meissner’s corpuscles, and many contain loops of tiny blood vessels. The extensive network of blood vessels in the dermal papillae plays an important role in the regulation of body temperature. The blood vessels dilate in hot environments to help dissipate heat, and they constrict to conserve heat in cold environments. Approximately one-fourth of the body’s blood flows through the skin at any given time.

The lower layer of the dermis is called the reticular layer. It is made primarily of coarse collagen and elastic fibers. Skin appendages such as glands and hair follicles are often anchored in the reticular layer of the dermis. The reticular layer also contains several different types of sensory receptors, nerve cells specialized to detect various stimuli, including pain, heat, cold, itch, and pressure. For example, Pacinian corpuscles are receptors found in the deep dermis of weight-bearing surfaces, such as the soles of the feet. They are composed of concentric layers of cells, much like an onion, and transmit vibrational stimuli. Sensory receptors are more dense in hairless areas, such as the fingertips and lips, making these areas especially sensitive.




In humans, the skin appendages, or structures embedded in the skin, include hair, nails, and several types of glands. Glands are groups of cells that produce and secrete substances needed by other parts of the body. In other vertebrates, the skin appendages include scales (in fish and reptiles) and feathers (in birds). Together, the skin and the skin appendages are known as the integumentary system of the body.




Hair Growth

A hair grows upward from the root. Lengthening fibers of keratin-filled dead cells, grouped around the semihollow medulla, make up the cortex. A living structure called the bulb (visible as a white lump at the end of a plucked hair) surrounds and feeds the root, which lies in a pocket of the epidermis called the follicle. Hair grows fastest when it is short.

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Hair is a distinguishing characteristic of mammals, a group of vertebrates that includes humans. A thick coat of body hair known as fur protects many mammals from the cold and from the sun’s ultraviolet rays. In humans, a species whose body hair is relatively sparse, this protective function is probably minimal, limited chiefly to the hair on the scalp.

Hair is composed primarily of keratin. The protein is packed into dead keratinocytes, much like those found in the upper layers of the epidermis. The dead keratinocytes fuse together to form the hair. The portion of the hair above the skin is known as the shaft, while that below the surface of the skin is known as the root. Each hair grows from its own follicle, an indentation of the epidermis. At the base of the follicle is the bulb, which contains cells that give rise to the keratinocytes that make up the hair, as well as blood vessels that nourish the growing hair. Hair on the scalp typically grows at a rate of 1 mm (0.04 in) every three days.

Each hair follicle also contains the arrector pili, a muscle that contracts in response to cold, fright, and other emotions. When the muscle contracts, it pulls the hair in the follicle into a vertical position. This response may help some mammals keep warm or look bigger to frighten or intimidate their enemies. But in humans, again because of our sparse coat of body hair, it merely produces “goose bumps.”

The color of hair is due to melanin, produced by melanocytes in the bulb of the hair follicle and then incorporated into the keratinocytes that form the hair. Dark hair contains true melanin like that found in the skin, while blond and red hair result from types of melanin that contain sulfur and iron. Hair goes gray when melanocytes age and lose the enzyme necessary to produce melanin. White hair occurs when air bubbles become incorporated into the growing hair. The texture of hair results from the shape of the hair shaft. Straight hair appears round in cross section, wavy hair has an oval shape in cross section, and the cross section of a curly hair has an elliptical or kidney-shaped appearance.





Human fingernails and toenails are made of dead cells containing the protein keratin. Nails have three parts: the concealed root, the body, which is exposed but attached to skin, and the edge. The nail grows out from the addition of new cells at the root.

Phil Jude/Science Source/Photo Researchers, Inc.

Nails on the fingers and toes are made of hard, keratin-filled epidermal cells. They protect the ends of the digits from injury, help us grasp small objects, and enable us to scratch. The part of the nail that is visible is called the nail body, and the portion of the nail body that extends past the end of the digit is called the free edge. Most of the nail body appears pink because of blood flowing in the tissue underneath, but at the base of the body is a pale, semicircular area called the lunula. This area appears white due to an underlying thick layer of epidermis that does not contain blood vessels. The part of the nail that is buried under the skin is called the root. Nails grow as epidermal cells below the nail root and transform into hard nail cells that accumulate at the base of the nail, pushing the rest of the nail forward. Fingernails typically grow 1 mm (0.04 in) per week. Toenails generally grow more slowly.




An adult human has between 1.6 million and 4 million sudoriferous glands, or sweat glands. Most are of a type known as eccrine sweat glands, which are found almost all over the surface of the body and are most numerous on the palms and soles. Eccrine sweat glands begin deep in the dermis and connect to the surface of the skin by a coiled duct. Cells at the base of the gland secrete sweat, a mixture of water, salt, and small amounts of metabolic waste products. As the sweat moves along the duct, much of the salt is reabsorbed, preventing excessive loss of this vital substance. When sweat reaches the outer surface of the skin, it evaporates, helping to cool the body in hot environments or during physical exertion. In addition, nerve fibers that encircle the sweat glands stimulate the glands in response to fear, excitement, or anxiety. The sweat glands can secrete up to 10 liters (2.6 gallons) of fluid per day, far more than any other type of gland in the body.

Other sweat glands, known as apocrine sweat glands, are much less numerous than eccrine sweat glands. Apocrine sweat glands are located mainly in the armpit, genital area, and around the nipples of the breasts. Apocrine sweat glands are also anchored deep in the dermis, but they open into hair follicles rather than onto the surface of the skin. They secrete a limited amount of a milky fluid—thicker than the secretion of the eccrine sweat glands—that is thought to be involved in sexual attraction in many mammal species.

Oil, or sebaceous, glands are found all over the body except on the palms, the soles, and the top of the feet. They are most numerous on the face and scalp. Most sebaceous glands open into hair follicles, but the glands also occur in some hairless areas, such as the lips and inside the mouth. Glands of this type produce an oily substance called sebum, which keeps the skin and hair from drying out and inhibits the growth of certain harmful bacteria.

Wax, or ceruminous, glands are located in the ear canal. They secrete a waxy substance that helps prevent foreign particles from entering the ear. Ceruminous glands are modified sweat glands.









Preliterate Societies

Trepanned Skull

Trepanning, the procedure of cutting a hole in the skull, is the earliest known medical operation. Some anthropologists believe that trepanning was performed on people with mental illnesses to drive out evil spirits from their heads. This skull dates from the Inca civilization.

Daniele Pellegrini/Photo Researchers, Inc.

Evidence for trepanning, the surgical procedure of cutting a hole in the skull, dates back 4,000 to 5,000 years. Some anthropologists speculate that Stone Age societies performed trepanning on people with mental illnesses to release evil spirits or demons from their heads. In the absence of written records, however, it is impossible to know why the operation was performed.



Ancient Societies


The Greek physician Hippocrates was one of the first scholars to challenge the notion that disease was punishment sent from the gods. He believed that all illnesses, including mental illnesses, had natural origins.

Culver Pictures

The literature of ancient Greece and Rome contains evidence of the belief that spirits or demons cause mental illness. In the 5th century bc the Greek historian Herodotus wrote an account of a king who was driven mad by evil spirits. The legend of Hercules describes how, driven insane by a curse, he killed his own children. The Roman poets Virgil and Ovid repeated these themes in their works. The early Babylonian, Chinese, and Egyptian civilizations also viewed mental illness as possession, and used exorcism—which sometimes involved beatings, restraint, and starvation—to drive the evil spirits from their victim.

Not all ancient scholars agreed with this theory of mental illness. The Greek physician Hippocrates believed that all illnesses, including mental illnesses, had natural origins. For example, he rejected the prevailing notion that epilepsy had its origins in the divine or sacred, viewing it as a disease of the brain. Hippocrates classified mental illnesses into categories that included mania, melancholia (depression), and phrenitis (brain fever), and he advocated humane treatment that included rest, bathing, exercise, and dieting. The Greek philosopher Plato, although adhering to a somewhat supernatural view of mental illness, believed that childhood experiences shaped adult behaviors, anticipating modern psychodynamic theories by more than 2000 years.



The Middle Ages

The Middle Ages in Europe, from the fall of the Roman empire in the 5th century ad to about the 15th century, was a period in which religious beliefs, specifically Christianity, dominated concepts of mental illness. Much of society believed that mentally ill people were possessed by the devil or demons, or accused them of being witches and infecting others with madness (see Witchcraft). Thus, instead of receiving care from physicians, the mentally ill became objects of religious inquisition and barbaric treatment. On the other hand, some historians of medicine cite evidence that even in the Middle Ages, many people believed mental illness to have its basis in physical and psychological disturbances, such as imbalances in the four bodily humors (blood, black bile, yellow bile, and phlegm), poor diet, and grief.

The Islamic world of North Africa, Spain, and the Middle East generally held far more humane attitudes toward people with mental illnesses. Following the belief that God loved insane people, communities began establishing asylums beginning in the 8th century ad, first in Baghdād and later in Cairo, Damascus, and Fez. The asylums offered patients special diets, baths, drugs, music, and pleasant surroundings.



The Renaissance

The Renaissance, which began in Italy in the 14th century and spread throughout Europe in the 16th and 17th centuries, brought both deterioration and progress in perceptions of mental illness. On the one hand, witch-hunts and executions escalated throughout Europe, and the mentally ill were among those persecuted. The infamous Malleus Maleficarum,which served as a handbook for inquisitors, claimed that witches could be identified by delusions, hallucinations, or other peculiar behavior. To make matters worse, many of the most eminent physicians of the time fervently advocated these beliefs.

On the other hand, some scholars vigorously protested these supernatural views and called renewed attention to more rational explanations of behavior. In the early 16th century, for example, the Swiss physician Paracelsus returned to the views of Hippocrates, asserting that mental illnesses were due to natural causes. Later in the century, German physician Johann Weyer argued that witches were actually mentally disturbed people in need of humane medical treatment.



The Age of Enlightenment

Pinel Frees the Insane

French physician Philippe Pinel supervises the unchaining of mentally ill patients in 1794 at La Salpêtrière, a large hospital in Paris. Pinel believed in treating mentally ill people with compassion and patience, rather than with cruelty and violence. This painting, Pinel Frees the Insane from Their Chains, was completed by French artist Tony Robert-Fleury in 1876.

Belzeaux/Photo Researchers, Inc.

During the Age of Enlightenment, in the 18th and early 19th centuries, people with mental illnesses continued to suffer from poor treatment. For the most part, they were left to wander the countryside or committed to institutions. In either case, conditions were generally wretched. One mental hospital, the Hospital of Saint Mary of Bethlehem in London, England, became notorious for its noisy, chaotic conditions and cruel treatment of patients (see Bedlam).


The Hospital of Saint Mary of Bethlehem, a London mental hospital commonly known as Bedlam, sold admission tickets to the public in the 18th century, becoming a popular tourist attraction. In this engraving by English artist William Hogarth, part of his series A Rake’s Progress (1735), two women (seen in the background) tour the hospital, watching the mentally ill patients for their amusement. The hospital became notorious for its miserable conditions and cruel treatment of patients.

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Yet as the public’s awareness of such conditions grew, improvements in care and treatment began to appear. In 1789 Vincenzo Chiarugi, superintendent of a mental hospital in Florence, Italy, introduced hospital regulations that provided patients with high standards of hygiene, recreation and work opportunities, and minimal restraint. At nearly the same time, Jean-Baptiste Pussin, superintendent of a ward for “incurable” mental patients at La Bicêtre hospital in Paris, France, forbade staff to beat patients and released patients from shackles. Philippe Pinel continued these reforms upon becoming chief physician of La Bicêtre’s ward for the mentally ill in 1793. Pinel began to keep case histories of patients and developed the concept of “moral treatment,” which involved treating patients with kindness and sensitivity, and without cruelty or violence. In 1796 a Quaker named William Tuke established the York Retreat in rural England, which became a model of compassionate care. The retreat enabled people with mental illnesses to rest peacefully, talk about their problems, and work. Eventually these humane techniques became widespread in Europe.



Reform in the United States

Clifford Beers

In 1908, after his release from a mental asylum, Clifford Whittingham Beers wrote A Mind That Found Itself, which exposed the poor conditions he had suffered while confined. He went on to establish several organizations dedicated to the promotion of mental health reforms in the United States.

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People living in the colonies of North America in the 17th and 18th centuries generally explained bizarre or deviant behavior as God’s will or the work of the devil. Some people with mental illnesses received care from their families, but most were jailed or confined in almshouses with the poor and infirm. By the mid-18th century, however, American physicians came to view mental illnesses as diseases of the brain, and advocated specialized facilities to treat the mentally ill. The Pennsylvania Hospital in Philadelphia, which opened in 1752, became the first hospital in the American colonies to admit people with mental illnesses, housing them in a separate ward. However, in the hospital’s early years, mentally ill patients were chained to the walls of dark, cold cells.

In the 1780s American physician Benjamin Rush instituted changes at the Pennsylvania Hospital that greatly improved conditions for mentally ill patients. Although he endorsed the continued use of restraints, punishment, and bleeding, he also arranged for heat and better ventilation in the wards, separation of violent patients from other patients, and programs that offered work, exercise, and recreation to patients. Between 1817 and 1828, following the examples of Tuke and Pinel, a number of institutions opened that devoted themselves exclusively to the care of mentally ill people. The first private mental hospital in the United States was the Asylum for the Relief of Persons Deprived of the Use of Their Reason (now Friends Hospital), opened by Quakers in 1817 in what is now Philadelphia. Other privately established institutions soon followed, and state-sponsored hospitals—in Kentucky, New York, Virginia, and South Carolina—-opened beginning in 1824.

Dorothea Dix

American reformer Dorothea Dix championed the causes of prison inmates, the mentally ill, and the destitute. Horrified by the conditions provided for the mentally ill in Massachusetts, Dix successfully petitioned the state government for improvements in 1843. She was directly responsible for building or enlarging 32 mental hospitals in North America, Europe, and Japan.


Nevertheless, circumstances for most mentally ill people in the United States, especially those who were poor, remained dreadful. In 1841 Dorothea Dix, a Boston schoolteacher, began a campaign to make the public aware of the plight of mentally ill people. By 1880, as a direct result of her efforts, 32 psychiatric hospitals for the poor had opened. Increasingly, society viewed psychiatric institutions as the most appropriate form of care for people with mental illnesses. However, by the late 19th century, conditions in these institutions had deteriorated. Overcrowded and understaffed, psychiatric hospitals had shifted their treatment approach from moral therapy to warehousing and punishment. In 1908 Clifford Whittingham Beers aroused new concern for mentally ill individuals with the publication of A Mind That Found Itself, an account of his experiences as a mental patient. In 1909 Beers founded the National Committee for Mental Hygiene, which worked to prevent mental illness and ensure humane treatment of the mentally ill.



Deinstitutionalization Movement

Following World War II (1939-1945), a movement emerged in the United States to reform the system of psychiatric hospitals, in which hundreds of thousands of mentally ill persons lived in isolation for years or decades. Many mental health professionals—seeing that large state institutions caused as much, if not more, harm to patients than mental illnesses themselves—came to believe that only patients with severe symptoms should be hospitalized. In addition, the development in the 1950s of antipsychotic drugs, which helped to control bizarre and violent behavior, allowed more patients to be treated in the community. In combination, these factors led to the deinstitutionalization movement: the release, over the next four decades, of hundreds of thousands of patients from state mental hospitals. In 1950, 513,000 patients resided in these institutions. By 1965 there were 475,000, and by 1990 state mental hospitals housed only 92,000 patients on any given night. Many patients who were released returned to their families, although many were transferred to questionable conditions in nursing homes or board-and-care homes. Many patients had no place to go and began to live on the streets.

The National Mental Health Act of 1946 created the National Institute of Mental Health as a center for research and funding of research on mental illness. In 1955 Congress created a commission to investigate the state of mental health care, treatment, and prevention. In 1963, as a result of the commission’s findings, Congress passed the Community Mental Health Centers Act, which authorized the construction of community mental health centers throughout the country. Implementation of these centers was not as extensive as originally planned, and many people with severe mental illnesses failed to receive care of any kind.



Recent Developments

One of the most important developments in the field of mental health in the United States has been the establishment of advocacy and support groups. The National Alliance for the Mentally Ill (NAMI), one of the most influential of these groups, was founded in 1972. NAMI’s goal is to improve the lives of people with severe mental illnesses and their families by eliminating discrimination in housing and employment and by improving access to essential treatments and programs.

During the 1980s, all levels of government in the United States cut back on funding for social services. For example, the Social Security Administration discontinued benefits for approximately 300,000 people between 1981 and 1983. Of these, an estimated 100,000 were people with mental illnesses. Although the government eventually restored Social Security benefits to many of these people, the interruption of services caused widespread hardship.

The emergence of managed care in the 1990s as a way to contain health care costs had a tremendous impact on mental health care in the United States. Health insurance companies and health maintenance organizations increasingly scrutinized the effectiveness of various psychotherapies and drug treatments and put stricter limits on mental health care. In response to these restrictions, Congress passed the Mental Health Parity Act of 1996. This law required private medical plans that offer mental health coverage to set equal yearly and lifetime payment limits for coverage of both mental and physical illnesses.

In 1997 the U.S. Equal Employment Opportunity Commission issued new guidelines intended to prevent discrimination against people with mental illnesses in the workplace. The rules, based on the Americans with Disabilities Act of 1990, prohibit employers from asking job applicants if they have a history of mental illness and require employers to provide reasonable accommodations to workers with mental illnesses.

In recent years international agencies, led by the World Health Organization (WHO) of the United Nations (UN) have developed mental health policies that seek to reduce the huge burden of mental illness worldwide. These agencies are working to improve the quality of mental health services in Africa, Asia, Latin America, the Middle East, and elsewhere by educating governments on prevention and treatment of mental illness and on the rights of the mentally ill.

Contributed By:
Alex Cohen
Arthur M. Kleinman

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