=Puberty.=—This is the period of transformation from childhood to
adolescence. There is a marked development of the throat and chest, the
breasts become fuller, more rounded, and there is also an enlargement
of the pelvis. These outward signs are combined with certain internal
changes, affecting especially the ovaries and the uterus, that are
described under the terms of ovulation and menstruation.
=Ovulation.=—The ovaries produce ova that are retained in cavities
termed ovisacs (Graafian follicles), which are scattered throughout the
ovary. Sometimes an ovisac contains more than one ovum, but usually
each ovisac contains but one. When the ovary becomes functionally
active, some of the ovisacs in the deeper part of the organ increase
in size and develop toward the surface. From time to time one of
these ovisacs, having continued to increase in size so as to form a
projection on the surface of the ovary, actually bursts and sets free
an ovum. This frequently occurs in association with menstruation,
but may take place at other periods. While this development of the
ovisac has been taking place there is a special flow of blood to the
pelvic contents, and the fringe-like processes of the Fallopian tube
become more closely applied, so that a cup is formed by the fimbriated
extremity of the Fallopian tube, into which the ovum drops. Eventually
the ovum reaches the cavity of the uterus, and, unless it becomes
fertilized or impregnated, it is in due course cast off from the uterus
in the menstrual discharge and perishes.
When the uterus has become the seat of a fertilized ovum there is no
further ripening of ovisacs. After the escape of an ovum from the
ovisac the follicle is filled with blood, and this takes part in the
formation of a structure known as the corpus luteum, which is formed
within the burst ovisac. The corpus luteum varies in its development
according as the ovum becomes fertilized or not. If pregnancy takes
place it remains to its close, but if the ovum is not fertilized it
disappears in about three months. The ruptured ovisac always contains
the elements of a corpus luteum, but its development depends on
the fate of the ovum. Ovulation has been said to commence before
menstruation; ovisacs may sometimes ripen and burst in childhood.
Conception may take place before menstruation has set in.
=Menstruation.=—The blood is derived from the mucous membrane of the
uterus (endometrium) and not from that of the cervix. The endometrium
becomes swollen and thickened prior to the commencement of the flow,
and the blood-vessels are distended. With the beginning of the flow
this swelling gradually diminishes. A few superficial cells of the
endometrium are usually cast off during menstruation, and this is
regenerated during the interval between the periods. The mucous
membrane of the uterus is thus never at rest until menstruation ceases
for good.
=Conception.=—Fertilization of the ovum may occur anywhere in its
course from the ovisac to the uterus, but ordinarily it takes place in
the uterus. Certain changes take place in the uterus preparatory to the
arrival of the fertilized ovum. The whole organ enlarges, it contains
more blood than before; the mucous membrane becomes soft, spongy,
thickens greatly, and is thrown into folds which are called the decidua
vera.
=Decidua Vera.=—The part of the decidua vera to which the ovum becomes
attached and where the placenta afterward forms is called the decidua
serotina. The presence of the ovum acts as an irritant to the spot
where it lodges, and active growth takes place. At the same time the
ovum burrows into the mucosa and gradually becomes embedded. The vera
closes over the ovum. The decidua vera is a mold of the interior of
the uterus. The external surface is rough where it has been torn from
the underlying tissues of the uterus, and the internal aspect which is
in contact with the ovum is smooth. It is thickest at the third month;
after this time the membrane becomes thinner and thinner, until by
the end of pregnancy it is very thin. It is to a considerable extent
expelled at labor and forms part of the membranes.
=Ovum.=—As the ovum develops the decidua reflexa expands until it fills
the decidua vera at the end of the second month, and by the end of the
third month it has intimately united with the decidua vera to form one
membrane. Until these two membranes have coalesced the ovum is not free
in the uterine cavity, but is enclosed in the decidua reflexa.
=Pregnant Uterus.=—As pregnancy proceeds the uterus ceases to be a
pelvic organ; it becomes rounded, and after the sixth month gradually
assumes an ovoid shape. At the end of the third month it has reached
to the brim of the pelvis. By the sixth month it is level with the
umbilicus, and by the ninth it has reached to the tip of the sternum.
During the last fortnight the organ sinks a little, and this relieves
to some extent the shortness of breath which has been caused by the
pressure on the diaphragm.
=Fetal Membranes.=—Inside of the decidual membranes we find the chorion
and amnion. Whereas the deciduæ are maternal structures, the chorion
and the amnion are derived from the ovum. The chorion is the more
external of the two fetal membranes; it early surrounds the ovum and
throws out processes (villi) toward the decidua reflexa and serotina.
These processes are at first formed equally all over the chorion, but
those which lie in connection with the decidua serotina grow more
rapidly in size and complexity, and remain to form the main part of the
placenta. The membrane in which the fetus lies is the amnion. Contained
within the amnion is more or less fluid, the liquor amnii, in which
the fetus rests as in a water-bed. It permits free movements on the
part of the fetus and protects it from violence from the outside; it
equalizes the pressure on the uterine walls, preventing pressure on
the cord and placenta; and during labor it assists in the dilation of
the os uteri and lubricates the maternal passages. From it also the
fetus derives its supply of water. Occasionally water may form between
the three membranes, and during labor the chorion may rupture and not
the amnion, and the waters between them be discharged. In like manner,
water which has formed between the decidua and the chorion may come
away. These, then, are two sources of fluid which may be discharged
from the vagina in labor when the amnion is intact.
=Umbilical Cord.=—The organ by which the fetus is attached to the
uterus is a whitish, glistening structure which springs from the fetus
at the center of the abdomen, and passes to the internal aspect of
the placenta. It contains two arteries and one vein surrounded by a
gelatinous material (Wharton’s jelly), the whole being covered with
a layer derived from the amnion. The vessels usually run tortuously
in the cord, thus lessening the danger of compression, which might
otherwise occur. This arrangement also tends to regulate and equalize
the circulation of the blood through its vessels.
=Placenta.=—The “after-birth” is a spongy cavernous organ and has a
circumference of from 20 to 24 inches. It is made up of two elements,
fetal and maternal, chiefly the former. The fetal surface, to which
the umbilical cord is attached, is formed by the amnion. External to
the latter is the chorionic tissue, consisting mainly of finger-like
processes of villi, many of which extend to the decidua. These villi
are mostly vascularized. The vascular villus is made of a capillary
loop, lying embedded in a connective tissue, the blood-vessels being
derived from the umbilical vessels. The villi are surrounded by
maternal blood, which circulates in the intervillous spaces. The
maternal part is represented by the decidua serotina. Its surface is
rough and irregular, being broken up into segments or cotyledons. It
is made of two layers: (_a_) The deep or spongy layer, and (_b_) the
compact, or layer to which the villi are attached. The decidua serotina
contains large cavities or sinuses, which are formed by a dilation
of the blood-vessels in the wall of the uterus. They communicate
with the intervillous spaces of the placenta, and it is through the
maternal blood circulating in them that nutriment is conveyed to the
fetus through the medium of the villi. An interchange takes place
between the fetal and maternal bloodstreams, though, of course, there
is no direct continuity between them. The placenta thus has a two-fold
function in acting both as an organ of circulation and of respiration.
It is through the walls of the villi that this interchange of gases
and fluids takes place; nourishment and oxygen passing through them to
the fetus, and carbonic-acid gas and waste materials passing through
them to the mother. There is no direct communication between the fetal
and the maternal blood. The placenta, normally, is attached to the
contractile part of the uterus, and on its separation the blood-vessels
are closed by the contractions of the uterine wall. The placenta may
be divided into two or more portions, or one or more of its cotyledons
may be detached from the rest of the organ and have their own vascular
supply. The importance of this lies in the fact that after the placenta
is expelled these isolated masses may be left and give rise to
hemorrhage or blood-poisoning.