Nipples ;)

In its
most
general form, a nipple is a
structure from which a
fluid emanates. More
specifically, it is the
projection on the breasts
or udder of a mammal by
which breast milk is
delivered to a mother's
young. In this sense, it is
often called a teat,
especially when referring
to non-humans, and the
medical term used to refer
to it is papilla. The rubber
mouthpiece of a baby
bottle or pacifier may also
be referred to as a
"nipple" or a "teat". In
many cultures, female
nipples are considered an
erogenous zone and it is
considered a public
indecency to uncover
them in public.
The word "nipple" most
likely originates as a
diminutive of neb, an Old
English word meaning
"beak", "nose", or "face",
and which is of Germanic
origin.[1] The words "teat"
and "tit" share a Germanic
ancestor. The second of
the two, tit, was inherited
directly from Proto-
Germanic, while the first
entered English via Old
French.[2][3]
Anatomy
In the anatomy of
mammals, a nipple,
mammary papilla or teat
is a small projection of
skin containing the outlets
for 15–20 lactiferous ducts
arranged cylindrically
around the tip. The skin of
the nipple is rich in a
supply of special nerves
that are sensitive to
certain stimuli. The
physiological purpose of
nipples is to deliver milk
to the infant, produced in
the female mammary
glands during lactation.
Marsupials and eutherian
mammals typically have
an even number of nipples
arranged bilaterally, from
as few as two to as many
as 19.[4] They develop in
the embryo, along the
'milk lines'. Most mammals
develop multiple nipples
along each milk line, with
the total number
approximating the
maximum litter size, and
half the total number (i.e.
the number on one side)
approximating the
average litter size for that
species. Monotremes, such
as the platypus, lack teats;
their young drink milk
directly from pores in the
skin (similar to sweat
glands), or by sucking it
off of hairs surrounding
the pores.
In cetaceans such as
whales, the infant cannot
form a suction-seal to
nurse, due to its mouth
structure; the whale's
nipple is therefore unlike
that of any other
mammal. Rather than
requiring a sucking action,
the discharge of milk is
powered by maternal
muscles. The calf takes the
extended nipple into its
mouth, and the mother
ejects or expels her milk
into the mouth of the calf.
Teats vary in size, location,
and structure in different
mammalian species. Teats,
on occasion, become so
plump and filled, milk may
escape without suckling.
Female goats and ewes
have two teats, each with
a single mammary gland,
located between the
hindlegs. Mares have two
teats, each with two
mammary glands. The
teats of the sow can be
quite variable in number,
from six to 16, and are
located on two parallel
lines along the belly. Cows
have four teats, each with
one mammary gland in
the udder. Humans have
two teats. Extra teats
occur often, and are
known as supernumerary
teats. They are
nonfunctional.
In most eutherian
mammals, both males and
females have teats. In the
male, nipples are often not
considered functional with
regard to breastfeeding,
although male lactation is
possible. Notable
exceptions to this are rats
and horses, neither of
which has teats in the
male.[citation needed]
The pigments of the
nipple and areola are
brown eumelanin, a
brown pigment, and to a
greater extent
pheomelanin, a red
pigment. In many women,
there are small bulges on
the areola, which are
called " Montgomery
bodies". In humans, the
nipple is innervated by the
fourth intercostal nerve.
Instinct
Mammalian infants have a
"rooting" instinct (moving
their heads so as to bring
their mouths towards
whatever is touching their
faces) for seeking the
nipple, and a "suckling"
instinct for extracting milk.
The offspring of domestic
animals, including piglets,
calves, lambs, and foals,
engage in a behavior
known as teat seeking,
sipping, and suckling. This
strong instinct occurs in
most species within
minutes of birth, and
serves both to connect the
young to the food source
and to encourage bonding
between mother and
young. The offspring
thoroughly enjoy their
suckling time, and may
suckle even after filling up.
In humans
Infants
Most humans have two
nipples after birth, located
near the center of each
breast, which are
surrounded by an area of
sensitive, pigmented skin
known as the areola.
Human fetuses develop
several more nipples
along the milk lines, which
extend from the axilla
(armpit), along the
abdominal muscles, and
down to the pubis (groin)
on both sides. Those
nipples usually disappear
before birth, but
sometimes remain,
resulting in
supernumerary nipples,
which uncommonly have
lactiferous glands
attached. Sometimes,
babies, male or female, are
born producing milk. This
is common and is
colloquially called " witch's
milk". It is caused by
maternal estrogens acting
on the baby and
disappears after several
days.