reproductive

reproductive organs, female the ovaries, fallopian tubes, uterus, vagina, and vulva (external genitalia) of a female. The breasts are a secondary sex character, enclosing the mammary glands (see also Plates).



The ovaries link the reproductive system to the body's system of endocrine glands. Besides producing the ova, they secrete the female sex hormones estrogen and progesterone, which influence the body's development and general functioning as well as the sexual function. The two ovaries, each about the size of a small plum, lie one on each side of the pear-shaped uterus at its wide upper part. When a female is born, her undeveloped ovaries already contain the specialized cells that can eventually become ova. At puberty these ova begin to ripen, one a month; usually the ovaries alternate in producing them. As the undeveloped egg cell, called a follicle, begins to ripen, it makes its way to the ovary's surface, breaks through its own outer covering, and is released. Release of an egg cell, called ovulation, occurs about once every 28 days.

After its separation from the ovary the ovum is drawn into the nearby fallopian tube through its fringed, flared opening, and is moved along by rhythmic contractions of the tube's walls and by the cilia of its mucous membrane lining. In the course of its passage the ovum ripens fully, and if fertilization occurs it usually takes place while the ovum is moving through the fallopian tube.

The other end of the tube opens directly into the uterus, a muscular organ capable of stretching to contain a fertilized ovum growing through the 9 months of pregnancy. The mucous membrane lining the uterus is also specially adapted to hold the fetus securely and nourish it. When the ovum arrives, the hormones estrogen and progesterone produced in the ovary have previously stimulated the uterus to prepare its lining with extra blood vessels. If the egg has not been fertilized, it loses its vitality, the hormone supply ceases, and the extra blood and tissues are discharged from the body through the vagina, in the menstrual flow. If fertilization has taken place, the growth of a new life has begun; menstruation does not occur, and in fact ceases entirely during the 9 months (approximately 280 days) of pregnancy.

The lower end of the uterus forms an opening called the cervix (neck), which protrudes into the vagina. Enclosed by muscles and lined with mucous membrane, the vagina measures on the average about 8 cm (3 in) in length. In coitus it receives the penis (male copulatory organ) and the discharge of sperm during ejaculation. Like the uterus, the vagina undergoes changes during pregnancy that enable it to stretch to many times its usual size, allowing the infant to pass through it in childbirth.

The exterior opening of the vagina and the surrounding organs make up the vulva; this consists of the labia majora, labia minora, vestibule, and clitoris. Anterior to the vulva is a triangular fatty pad covered with pubic hair, the mons pubis. Between the clitoris and the entry to the vagina is the opening of the urethra, from which urine is excreted. The anus lies to the rear of the vaginal opening. In a virgin, a membrane called the hymen usually closes off a part of the opening to the vagina.

The labia majora envelop the labia minora, and these join together at the clitoris, a diminutive penislike organ that has a purely erotic function. Like the penis, the clitoris has a foreskin and many nerve endings. The vestibule is the area surrounding the entry to the vagina and within the labia minora; at each side of the vaginal opening and elsewhere in the vestibule, glands secrete lubricating fluids to facilitate coitus.

A woman's breasts serve to provide milk for the newborn infant. At puberty the breasts increase in size; during pregnancy they become much larger and start to secrete milk shortly after childbirth.

reproductive organs, male the external genitalia, accessory glands that secrete special fluids, and the accompanying ducts in the male (see also Plate 13).