It is very variable in form, but in the great majority of
instances it diminishes the size of the vaginal inlet to such an extent
as to render coitus impossible until the hymen has been torn. Through
the vaginal orifice access is gained to the interior of the _vagina_, a
tubular structure, but flattened from before backwards, so that in the
quiescent state the anterior and posterior walls of the passage are in
apposition. The _uterus_ or _womb_ is a muscular, pear-shaped organ,
with an elongated central cavity, which opens into the upper part of the
vagina. At the upper end of the cavity of the uterus are two small
laterally placed apertures, which lead into the _Fallopian tubes_ (or
_oviducts_). These tubes pass outwards in a somewhat sinuous course
towards the _ovaries_, the reproductive glands of the female, homologous
with the testicles in the male, and situated on either side of the upper
extremity of the uterus. The shape of the ovaries is somewhat ovoid.
They contain a large number of vesicular structures, the _ovarian
follicles_, the largest, ripe follicles being known as _Graafian
follicles_, whilst the smaller, partially developed follicles are termed
_primitive ovarian follicles_, or _primitive Graafian follicles_. In the
interior of each follicle is an _ovum_. In the sexually mature woman, a
Graafian follicle ripens at regular intervals of four weeks. When ripe,
the follicle bursts, the ovum is expelled, and passes through the
Fallopian tube into the interior of the uterus: here it is either
fertilised by uniting with a spermatozoon derived from the male, in
which case it proceeds to develop into an embryo; or else it remains
unfertilised, in which case it is shortly expelled from the body.

In the uterus, as well as in the ovaries, an important change occurs at
intervals of four weeks, characterised by an increased flow of blood to
the organ, culminating in an actual outflow of blood from the vessels
into the uterine cavity, and thence through the vagina to the exterior
of the body; the whole process is known as _menstruation_, the _monthly
sickness_ or the (_monthly_) _period_. After the fertilisation of the
ovum, during pregnancy, that is to say, menstruation usually ceases
until after the birth of the child, and often until the completion of
lactation.

I do not propose to discuss here the nature of the connexion between
these periodic processes in the ovaries and the uterus,
respectively--that is, betw