The secondary sex organs of the female serve to transport and care for the ovum and to develop the new individual (embryo and fetus).
a. Uterine Tube (Oviduct, Fallopian Tube). The uterine tube picks up the free ovum when it is expelled from the follicle of the ovary. The ovum stays in the uterine tube to await fertilization. If it is fertilized, it goes through the initial stages of embryonic development, and the embryo then passes on to the uterus. On the other hand, if it is not fertilized, its stored food is exhausted in 3 to 5 days; it dies and its remains are absorbed by the uterine tube.
b. Uterus. The uterus is a single pear-shaped organ located within the pelvic cavity of the female. The early embryo passes into the uterus from the uterine tube. The embryo continues its development within the uterus.
(1) Endometrium. The inner lining of the uterus is known as the endometrium. The endometrium is an epithelium containing uterine glands and blood vessels. Under the influence of the estrogens and progesterone, the embryo present at the end of the menstrual cycle, the endometrium breaks down. (This produces a "flow" of blood and cellular elements (menses) in a process known as menstruation.)
(2) Amniotic sac and placenta. When the embryo passes into the uterine cavity from the uterine tube, it "burrows" into the endometrium. Later, a fluid-filled sac (the amniotic sac) surrounds the embryo. The embryo floats free, surrounded by amniotic fluid. The embryo has an umbilical cord that originates in the center of its anterior abdomen. The umbilical cord is attached to the wall of the uterus by a special structure known as the placenta.
(3) Cervix. The cervix, the inferior end of the uterus, is inserted into the top of the vagina. Through the center of the cervix is the cervical canal. Its wall consists primarily of circular muscle tissue, which holds the opening closed until time for parturition (giving birth). During the initial stage of parturition, the cervical musculature dilates (stretches) to form an opening for the passage of the newborn (to be).
c. Vagina. The vagina is a tubular structure that extends from the cervix of the uterus to the exterior of the perineum. After the vagina receives the male penis, the semen is discharged into the upper recess opposite the opening of the cervix. At parturition, the vagina forms the birth canal through which the newborn passes to the outside.
d. External Genitalia. The opening of the vagina and of the urethra are covered by the external genitalia. Included among the external genitalia are two pairs of folds--the major and minor labia. Also included is the clitoris, a small structure comparable to the male penis but without the urethra.
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