sion of the ovule and of its
fecundation, as well as the changes in the womb which result from
these phenomena.

=Menstruation.=--About every four weeks, one or two ovules (rarely
more) mature and are discharged into the Fallopian tubes, down which
they pass by the movement of the vibratile cilia of the mucous
membrane, to the uterus, to the walls of which they become attached if
they have been fecundated on the way (Fig. 18). Fecundation or
conjugation takes place most often in the Fallopian tube, sometimes in
the uterus. The maturation and expulsion of the ovule are generally
accompanied in women by a nervous phenomenon closely related to
erection in man. The mucous membrane of the cavity of the uterus is
very rich in blood vessels which become dilated and gorged with blood
under the inhibitory influence of certain nerve centers. As the mucous
membrane is very thin, the result is otherwise than in man; the blood
transudes through the mucous membrane and flows away. This is called
_menstruation_ ("courses" or monthly periods). The object of this is,
no doubt, to prepare the mucous membrane of the womb for the fixation
of the fecundated egg which will become grafted on its surface. The
courses in women generally last three or four days, but are often very
irregular. It is necessary to point out that they do not depend on
ovulation (expulsion of the egg). The two phenomena may take place
independently of each other, for menstruation in itself depends only
on nervous irritation, which may be provoked or averted by hypnotic
suggestion, for example.

Moreover, there are women who never menstruate and who, in spite of
this, not only regularly discharge ovules but may be fecundated and
become pregnant. Usually, however, the two phenomena are associated by
nervous reflexes, so that menstruation takes place first and then the
ovule commences its migration.

=The Mechanism of Coitus.=--Copulation, or coitus, takes place as
follows: After a certain degree of excitation, both mental and
sensory, the male introduces the erect and stiffened penis into the
vagina. In the case of advanced pregnancy he should place himself
behind, so as to avoid injuring the unborn child. Rhythmic movements
of the two individuals, especially of the man, gradually increase the
excitation of the mucous membrane or skin of the genital organs of
each party, till voluptuous sensations, arising chiefly in the glans
penis and clitoris, spread to the whole