Second
part
The
Vagina and Internal Sex Organs
The
uterus (womb) is a hollow muscular organ shaped like an inverted
pear somewhat flattened front to back. It is about 7.5
centimeters (3 inches) long and 5 centimeters (2 inches) wide.
Anatomically the uterus consists of several parts. The inside
lining of the uterus (the endometrium) and the muscular
component of the uterus (the myometrium) have separate and
distinct functions. The inner lining changes during the
menstrual cycle and is where a fertilized egg implants at the
beginning of pregnancy. The muscular
facilitates labor and delivery.
Both aspects of uterine function are regulated by
chemicals called hormones, which also play a part in the growth
of the uterus during pregnancy.
The
uterus is held loosely in place in the pelvic cavity by six
ligaments. The
angle of the uterus in relation to the vagina varies from woman
to woman; ordinarily, it is relatively perpendicular to the axis
of the vaginal canal, but in about 25 percent, it is tilted
farther forward. If
the uterus is rigidly fixed in position by scar tissue or
inflammation, it may be a source of pain during sexual activity,
requiring surgical correction.
The
fallopian tubes, or oviducts, begin at the uterus and extend
about 10 centimeters (4 inches) laterally. The far ends of the
Fallopian tubes are funnel-shaped and terminate in long
fingerlike extensions called fimbria, which hover near the
ovaries. The inside lining of the Fallopian tubes pick up eggs
produced and released by the nearby ovary and then serve as the
meeting ground for egg and sperm.
The
ovaries, or female gonads, are paired structures located on each
side of the uterus. About the size of unshelled almonds (about 3
x 2 x 1.5 centimeters, or 1.2 x 0.8 x 0.6 inches), they are held
in place by connective tissue that attaches to the broad
ligament of the uterus. The ovaries have two separate functions:
manufacturing hormones (most notably, estrogen and progesterone)
and producing and releasing eggs.
Before
a baby girl is born, development of future eggs begins in her
just-forming ovaries. About halfway through her mother’s
pregnancy, the girl’s ovaries contain 6 or 7 million future
eggs, most of which degenerate before birth. About 400,000
immature eggs are present in the newborn girl, and no new eggs
are formed after this time. During childhood, continued
degeneration reduces the number of eggs still further. The
immature eggs are surrounded by a thin capsule of tissue forming
a follicle.
When
puberty arrives and girls begin to have menstrual cycles each
cycle is marked by a process of maturation in which some
immature eggs divide twice, splitting their genetic material in
half. Through this process, called meiosis, each young egg
divides into four cells, only one of which is a mature egg
(ovum). A mature egg is about 0.135 millimeters (1/175 inch) in
diameter and is surrounded by a zone of jellylike material
called the zona pellucida. A human egg is just barely visible,
appearing as a speck smaller than the period at the end of this
sentence. The other three cells, called polar bodies, have no
known function, and eventually degenerate.
Although
a number of different follicles begin growing in each cycle,
usually only one develops to the point where it moves to the
surface of the ovary and ruptures, releasing the egg in a
process called ovulation. For every follicle that ovulates,
about a thousand undergo various degrees of growth and then
degenerate. Fewer than 400 follicles are usually involved in
ovulation during the female’s reproductive years.
