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Variations in human hair
Human hair varies enormously. Different people have
hair that differs in color, in length, in diameter, and in its
distribution on the body. As we shall see later, some of these factors
are influenced by the person's racial type, and some by his or her
age.
Types of hair
Three types of hair grow on the human body.
Lanugo hair
This is the hair that develops on an unborn baby. It begins to grow
about three months after the baby's conception. The hairs are fine
and soft, and they grow all over the baby's body. They all grow
at the same rate, so the hairs are the same length.
Some prematurely born babies are still covered with these
downy hairs. Normally they are shed about four weeks before the
baby is due to be born.
Vellus hairs
Vellus hairs are short hairs, only a centimetre or two long,
and contain little or no pigment. The follicles that produce them
do not have
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oil glands (often called sebaceous glands), and
never produce any other kind of hairs.
Terminal hairs
Terminal hairs are the long hairs that grow on the head and
in many people on the body, arms and legs too. They are produced
by follicles with sebaceous glands. In people who have inherited
a tendency to baldness the hairs in these follicles gradually become
thinner and shorter until they look like vellus hairs.
Variation with age
Childhood
A newly born full-term baby has two types of hair. Terminal
hairs grow on the scalp and eyebrows, but nowhere else. All the
rest of the hair is vellus hair.
As the baby grows, the hair on the head grows too.
There are two periods during which hair grows rapidly on the scalp.
In both, the hair growth begins at the forehead and then extends
to the back of the neck. When the baby is two or three mdnths old,
the first hairs may be shed naturally over an area on the back of
the head. This is often mistakenly thought to be due to head rubbing;
hairs broken by rubbing may, however, be found on other parts of
the head as well.
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