ation
and extrusion of a ripe ovum; menstruation, which will be further
discussed in a separate chapter, consists in the monthly discharge of
blood, mixed with mucus from the inside lining of the uterus. Every
twenty-eight days, from the time of puberty to the time of the
menopause, a Graafian follicle bursts and an ovum is extruded from the
ovary. Before the follicle bursts, it swells and enlarges and reaches
the surface of the ovary; the whole follicle is congested with blood,
but at one point near the surface of the ovary it is pale and thin,
and here the rupture takes place.

[Illustration: SECTION OF OVARY.
1. Graafian follicle in the earliest stage. 2, 3, 4. Follicles in
more advanced stages. 5, 7. Almost mature follicle. 6. Follicle
from which the ovum has escaped. 8. Corpus luteum.]

=Corpora Lutea.= After the Graafian follicle has burst and the ovum
has been pushed out, the cavity that is left does not remain empty and
functionless; there is a further process going on there; there is a
growth of cells, of a yellowish color, and the follicle becomes filled
with a yellowish body, which on account of its color is called the
_corpus luteum_ (plural--corpora lutea; luteum in Latin--yellow,
corpus--body). This corpus luteum grows in size until it sometimes
occupies as much as one-third of the ovary. But there is considerable
difference between the corpora lutea of non-pregnant and pregnant
women. Up to the end of about a month the corpora lutea are the same,
but after that the corpus luteum of the non-pregnant woman begins to
get smaller, to shrink, so that at the end of two or three months it
is reduced to a small scar and later cannot be noticed at all. The
corpus luteum of the pregnant woman keeps on increasing until the end
of the second month, remains about the same size until the end of the
sixth month, and only then begins gradually to diminish. The corpus
luteum of the non-pregnant woman, that is, the one following
menstruation, is called false corpus luteum; the corpus luteum
following pregnancy is called a true corpus luteum. The corpus luteum
acts like a gland and elaborates a secretion which has an influence on
the circulation in the uterus and on menstruation. It probably
possesses other properties, with which we are not yet quite familiar.
The corpora lutea of various animals are now prepared in powder or
tablet form and used in medicine in the treatment of certain diseases
of women.