Hair

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Hair is a filamentous outgrowth of dead cells from the skin, found mainly in mammals.

In some species, it is absent at certain stages of life. It projects from the epidermis (skin), though it grows from hair follicles deep in the dermis. So-called "hairs" (trichomes) are also found on plants. The projections on arthropods, such as insects and spiders are actually insect bristles. The hair of non-human species is commonly referred to as fur. There are varieties of cats, dogs, and mice bred to have little or no visible hair.

Contents

In Humans

Types of hair

Humans have three different types of hair:

  • Lanugo, the fine hair that covers nearly the entire body of fetuses
  • Vellus hair, the short, fine, "peach fuzz" body hair that grows in most places on the human body in both sexes
  • Terminal hair, the fully developed hair, which is generally longer, coarser, thicker, and darker than vellus hair

Growth

Distribution of androgenic hair on female and male body

Different parts of the human body feature different types of hair. From childhood on vellus hair covers the entire human body regardless of sex or race except in the following locations: the lips, the nipples, the palms of hands, the soles of feet, certain external genital areas, the navel and scar tissue. The density of the hairs (in hair follicles per square centimeter) varies from one person to another.

The rising level of male hormones (androgens) during puberty causes a transformation process of vellus hair into terminal hair on several parts of the male body. The hair follicles respond to androgens, primarily testosterone and its derivatives; the hair in these locations can be thus termed androgenic hair. The rate of hair growth and the weight of the hairs increase. However, different areas respond with different sensitivities. As testosterone level increases, the sequence of appearance of androgenic hair reflects the gradations of androgen sensitivity. The pubic area is most sensitive, and heavier hair usually grows there first in response to androgens.

Layers of an individual hair

Areas on the human body that develop terminal hair growth due to rising androgens in both sexes, men and women, are the underarms and the pubic area. In contrast, normally only men grow androgenic hair in other areas. There is a sexual dimorphism in the amount and distribution of androgenic hair, with males having more terminal hair (particularly facial hair, chest hair, abdominal hair and hair on legs and arms) and females having more vellus hair, which is less visible. The genetic disposition determines the sex-dependent and individual rising of androgens and therefore the development of androgenic hair.

Increased body hair on women following the male pattern can be referred to as hirsutism. An excessive and abnormal hair growth on the body of males and females is defined as hypertrichosis. Considering an individual occurrence of body hair as abnormal does not implicitly depend on medical indications but also on cultural and social attitudes.

Individual hairs alternate periods of growth and dormancy. During the growth portion of the cycle, hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After three to six months, body hair growth stops (the pubic and armpit areas having the longest growth period). The follicle shrinks and the root of the hair grows rigid. Following a period of dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows for a long duration and to a great length before being shed. The rate of growth is approximately 1.25 centimeters, or about 0.5 inches, per month.

Texture

Hair texture is measured by the degree to which one's hair is either fine or coarse, which in turn varies according to the diameter of each individual hair. There are usually four major types of hair texture: fine, medium, coarse and wiry. Within the four texture ranges hair can also be thin, medium or thick density and it can be straight, curly, wavy or kinky. Hair conditioner will also alter the ultimate equation and can be healthy, normal, oily, dry, damaged or a combination. Also, an expert hairdresser can change the hair texture with the use of special chemicals.

Aging

Older people tend to develop grey hair because the pigment in the hair is lost and the hair becomes colourless. Grey hair is considered to be a characteristic of normal aging. The age at which this occurs varies from person to person, but in general nearly everyone 75 years or older has grey hair, and in general men tend to become grey at younger ages than women.

Width

According to The Physics Factbook, the diameter of human hair ranges from 17 to 181 µm.[1] (0.0007 to 0.007 inch)

Cultural attitudes

Head hair

The remarkable head hair of humans has gained an important significance in nearly all present societies as well as any given historical period throughout the world. The haircut has always played a significant cultural and social role.

In ancient Egypt head hair was often shaved, especially amongst children, as long hair was uncomfortable in the heat. Children were often left with a long lock of hair growing from one part of their heads, the practice being so common that it became the standard in Egyptian art for artists to depict children as always wearing this "sidelock". Many adult men and women kept their heads permanently shaved for comfort in the heat and to keep the head free of lice, while wearing a wig in public.

In ancient Greece and ancient Rome men and women already differed from each other through their haircuts. The head hair of women was long and pulled back into a chignon. Many dyed their hair red with henna and sprinkled it with gold powder, often adorning it with fresh flowers. Men’s hair was short and even occasionally shaved. In Rome hairdressing became ever more popular and the upper classes were attended to by slaves or visited public barber shops.


The traditional hair styling in some parts of Africa also gives interesting examples of how people dealt with their head hair. The Maasai warriors tied the front hair into sections of tiny braids while the back hair was allowed to grow to waist length. Women and non-warriors, however, shaved their heads. Many tribes dyed the hair with red earth and grease; some stiffened it with animal dung.

Contemporary social and cultural conditions have constantly influenced popular hair styles. From the 17th Century into the early 19th Century it was the norm for men to have long hair often tied-back into a ponytail. Famous long-haired men include Oliver Cromwell, George Washington and during his younger years Napoleon Bonaparte had a long and flamboyant head of hair. Before World War I men generally had longer hair and beards. The trench warfare between 1914 and 1918 exposed men to lice and flea infestations, which prompted the order to cut hair short, establishing a norm that has persisted.

However it has also been advanced that short hair on men has been enforced as a means of control, as shown in the military and police and other forces that require obedience and discipline. Additionally, slaves and defeated armies were often required to shave their heads, in both pre-medieval Europe and China.

Growing and wearing long hair is a lifestyle practiced by millions worldwide. It was almost universal among women in Western Culture until World War One. Many women in conservative Pentecostal groups abstain from trimming their hair after conversion. The social revolution of the 1960s led to a renaissance of unchecked hair growth. Hair length is measured (in inches or centimeters) from the front scalp line on the forehead up over the top of the head and down the back to the floor. Standard milestones in this process of hair growing are classic length (midpoint on the body, where the buttocks meet the thighs), waist length, hip length, knee length, ankle/floor length and even beyond. It takes about seven years, including occasional trims, to grow one's hair to waist length. Terminal length varies from person to person according to genetics and overall health. Large internet communities are set up to encourage and support a long hair lifestyle.

Hair style can also surpass personal expression and enter the realm of artist expression. A thriving salon culture in Detroit gave rise to the Detroit Hair Wars in 1991. Using the medium of human and synthetic hair, elaborate fantastical head pieces, such as spider webs, flowers and flying "hair-y copters", have been made by participants.[2] The nationally touring, spectacular showcase redefines the limits of head hair.

Body hair

The attitudes towards hair on the human body also vary between different cultures and times. In some cultures profuse chest hair on men is a symbol of virility and masculinity; other societies display a hairless body as a sign of youthfulness.

In ancient Egypt, people regarded a completely smooth, hairless body as the standard of beauty. An upper class Egyptian woman took great pains to ensure that she did not have a single hair on her body, except for the top of her head (and even this was often replaced with a wig. The ancient Greeks later adopted this smooth ideal, considering a hairless body to be representative of youth and beauty. This is reflected in Greek female sculptures which do not display any pubic hair. Islam stipulates many tenets with respect to hair, such as the covering of hair by women and the removal of armpit and pubic hair.

In Western societies it became a public trend during the late twentieth century, particularly for women, to reduce or to remove their body hair. The bikini fashion as well as the sexual imagery in advertising and movies are major reasons for this development. This media trend began in the United States and is becoming ever more popular throughout other Western countries. It is also beginning to gain currency among men, among whom shaving or trimming one's body hair is sometimes jokingly called "manscaping".

Hair as business factor

Hair care for humans is a major world industry with specialized tools, chemicals and techniques. The business of various products connected with human hair has become an important industrial and financial factor in Western societies.

See also

External links

Discuss



*Some information provided in whole or in part by http://en.wikipedia.org/

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