Hair: composition and features
Hair is decoration of any person and needs some care to look well. To keep
hair in good condition, it is necessary to know, what it is.
Structure of the hair shaft
Medullary substance;
Film |Cuticle;
Cortical substance
Hair
Capsula 3
cortex 2
Medulla 1
scarfskin
embryonic layer
Sebaceous glands
Papillary layer
Protective layer of fatty acids
epidermis
Reticular layer
Hair bag
Lipoblasts
the subcutis, a subcutaneous layer of fat
Muscle
Derma
Hair follicle
Fat cells
3% of hair is moisture and 97% is protein. A protein substance is keratin,
enriched with sulfur, microele-ments (iron, copper, zinc, chrome, manganese and
vitamins A, B, Р, С).
Hair grows over most of the body, except the palms of the hands and the soles of
the feet.
The average total number of hairs of adult is about 100,000, depending on hair
colour. Redheads have the densest hair, but they have the total number of hair
less than brunettes. Blondes have the biggest total number of hair -
approximately 150,000. Black people have the thickest hair, it can be 3 times
thicker than the hair of blondes.
Hair on the scalp grows irregularly: more on the top of the head and less on a
temporal part and on the forehead.
The first hairs begin growing at the end of the third month of fetal life.
Human hair grows at a different rate. On the scalp, hair grows at the fastest
rate (about 1 mm per 3 days), and it grows at the slowest rate on eyebrows.
The average life of different varieties of hair varies from several months up to
6 years. The eyelash life is from 3 up to 5 months, of downy hairs is 7-10
months. There is a small but steady molt of about 30 to 50 hairs a day.
Grown healthy hair is usually strong and elastic. The hair can be stretched for
1/5 of its length and after that it returns to its normal state. Hair is equal
to aluminum regarding fastness and capable to sustain a load from 100 up to 200
g. It is no wonder that in old times female hair was braided for making ropes
for lifting of big loads.
Hair is hygroscopic, that is—capable to take up moisture. It is rather steady
against influence of not strong acids, but has bad tolerance against alkaline
compositions. Hair plays the big role in human life. First, it is a perfect
decoration, emphasizing charm, hiding disadvantages, and, second, it fulfils
some important functions. Hair protects the head against excessive heat and cold.
Downy hair participates in sense of touch, eyelashes protect eyes, hair inside
the external ears and nostrils detain dust.
Hair has ability to accumulate some materials that enables to use it as an
identifier. Criminalists have been using this hair property successfully in
their work for a long time.
The part of hair protruding above the skin is called the shaft, and the part,
which is hidden beneath the skin surface, is called the root. At the end of the
root there is the hair bulb. The bulb manufactures new hairs. A hair papilla
from connective tissue juts out into the hair bulb and carries vessels for
nutrition of the hair bulb.
The hair shaft consists of three layers (diagram). The hair core called
medullary substance contains the keratinized cells. Medullary substance is
absent in downy hair and on the ends of other hair.
The basic mass of hair consists of cortical substance composed from extended
cells which fit tightly to each other and have the oblong nucleus or its traces,
containing a pigment: a colouring matter. The pig-ment can be red, yellow and
black, and combination of these colours creates individual hair colour. When
pigment is absent, hair is white.
The external layer of hair, a cuticle, consists of flat keratinized cells
without nucleus, which are arranged in one layer and overlay one after another
like a tile.
The hair root is located in the hair pocket, which opens as a small dilatation
on the skin. Excretory duct of sebaceous gland runs into the dermis follicle
approximately on the border of external third and middle third of hair.
The hair pocket consists from connective tissue and epithelium. The connective
tissue reaches its com-plete development only at the bottom part of the root,
beginning from the influx of sebaceous glands. Col-lagen fibers with admixture
of elastic fibers and argentophilic fibers form the external, longitudinal and
medium ring layer. The argentophilic fibers form a membrane, which fits tightly
to external epithelial layer of the follicle, around of the hair bag. The inner
layer, developed a little bit higher than the bulb, consists of very thin
homogeneous glassy membrane refracting the light strongly.
The epithelial part of the hair bag includes all layers of epidermis. Further in
the depth, the keratoid layer disappears.
The external layer is formed by one row of completely keratinized cells without
nucleus. Near the influx of sebaceous gland, cells of both layers become scaly
and, breaking up, they mix with fat of the gland.
All elements of hair components gradually become nuclear and conglomerate of
strenuously reproducing cells on the place of the bulb near a papilla. Hair is
manufactured from here.
Hair is provided with unstriped muscles (arrectores pilorum), looking like a
tape, which is attached by one end by means a short tendon to the compact layer
of derma and by the another end—to the place below the influx of the sebaceous
gland mouth. Contracting, the muscle raises hair and, compressing thus the
sebaceous gland, promotes excretion of its secret. Every person can experience
this process at least once in a lifetime. They say, feeling a strong fear: “His
heart was in his boots and the hair rose on his head”. The hair muscle actuates
this mechanism.
